Homepage weight change after moving to Shopify, by previous platform (medians per site)
| Previous platform | Image weight change | JavaScript change | Total weight change | Images lighter | Sites |
|---|---|---|---|---|---|
| BigCommerce | -1,330 KB | +890 KB | +203 KB | 74.7% | 91 |
| PrestaShop | -1,206 KB | +790 KB | +536 KB | 74.8% | 147 |
| Salesforce Commerce Cloud | -730 KB | +802 KB | +639 KB | 66.7% | 99 |
| WordPress | -654 KB | +599 KB | -155 KB | 70.1% | 87 |
| Magento | -628 KB | +519 KB | +286 KB | 69.9% | 382 |
| Nuxt | -473 KB | +402 KB | +66 KB | 67.7% | 31 |
| WooCommerce | -307 KB | +875 KB | +809 KB | 56.2% | 527 |
| Next.js | +109 KB | +330 KB | +401 KB | 42.9% | 119 |
Source: PageSpeed Matters, Sept 2026. Medians of per-site changes, so image and JavaScript changes do not add up to the total.
Summary
Stores that move to Shopify often expect a lighter, faster site. We wanted to know whether the homepage really gets lighter, and where the bytes go.
Images do get lighter. Median homepage image weight fell after every common ecommerce move to Shopify, from 307 KB lighter for stores leaving WooCommerce to 1,330 KB lighter for stores leaving BigCommerce. But JavaScript grew by more, so most stores still ended up with a heavier homepage overall.
What we measured
We used HTTP Archive monthly crawls of the top 1M sites from September 2024 to September 2026. A site counts as switching platform when it ran on one platform for 3 or more months and then on another for 3 or more months. This study covers sites that switched to Shopify.
For each store we averaged homepage bytes over months 2 to 4 before the switch and months 1 to 3 after it, then took the difference. The figures in this study are medians of those per-site differences, for image bytes, JavaScript bytes and total bytes.
Most platform moves come with a redesign, and those redesigns are included. The changes below are what happened to homepages after the move, not an effect of Shopify alone.
Image weight fell after every ecommerce move to Shopify
Median homepage image weight fell after every move to Shopify from another ecommerce platform. Stores leaving BigCommerce cut the most, at 1,330 KB (91 sites), followed by PrestaShop at 1,206 KB (147), Salesforce Commerce Cloud at 730 KB (99), Magento at 628 KB (382) and WooCommerce at 307 KB (527).
Images also shrank as a share of the page. They made up a median 46.9% of homepage bytes before the move and 23.9% after, so images went from close to half the homepage to under a quarter.
A likely reason is that Shopify's image CDN resizes images and serves modern formats, and a new theme often starts with fewer, smaller hero images. Because redesigns are part of the data, we cannot separate the two.
JavaScript growth outweighed the image savings
The image savings did not make most homepages lighter. Across 1,246 stores moving to Shopify from those five ecommerce platforms, median image weight fell 589 KB while JavaScript rose 773 KB, and total homepage weight still grew a median 539 KB.
The pattern holds store by store. 64.8% of these stores shipped fewer image bytes after the move, but 81.3% shipped more JavaScript and 57.2% shipped more total bytes.
The extra JavaScript likely comes from the theme and the apps a store installs for reviews, upsells, search, loyalty and tracking. Each one tends to add its own script, and they add up quickly on the homepage.
WooCommerce moves added the most weight
Moves from WooCommerce added the most total weight. The median store grew by 809 KB, with JavaScript up 875 KB against an image cut of only 307 KB. Fewer of these stores shipped lighter images, at 56.2% of 527.
Moves from WordPress, 87 sites, were the only group where total weight fell, by a median 155 KB. Images fell 654 KB, more than the 599 KB of added JavaScript.
Stores leaving BigCommerce cut the most image weight but still grew by a median 203 KB in total, as JavaScript rose 890 KB. Stores leaving Salesforce Commerce Cloud grew by 639 KB and stores leaving Magento by 286 KB.
Next.js moves and top stores break the pattern
Moves to Shopify from Next.js, 119 sites, were the exception on images: median image weight rose 109 KB, and only 42.9% of these stores shipped lighter images. Total weight grew a median 401 KB.
The most popular stores add the most JavaScript. Top 100K stores, 72 sites, cut images by the same median 589 KB as the wider group, but added 963 KB of JavaScript, for a median total increase of 1,239 KB. Bigger stores often run more apps, marketing tags and personalization, and those scripts likely account for much of the gap.
What this means for your store
If you are moving to Shopify, the image savings often come with the new theme and image CDN, but the JavaScript will not stay small without effort. Set a JavaScript budget for the homepage before launch, and measure it against your old site.
Add apps one at a time and check what each one loads. Remove apps you no longer use, since some leave scripts behind, and prefer theme features over apps where they do the same job. Load chat, reviews and other widgets only on the pages that need them.
After launch, compare your homepage in PageSpeed Insights or WebPageTest with the old version, looking at total bytes and JavaScript bytes, not just image weight.
Sources
- HTTP Archive monthly crawls, top 1M sites, September 2024 to September 2026
- web.dev, efficiently load third-party JavaScript: https://web.dev/articles/efficiently-load-third-party-javascript
Embed this research
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<p style="font:14px/1.5 system-ui,sans-serif">Source: <a href="https://www.pagespeedmatters.com/resources/data-studies/shopify-migration-image-weight">Shopify Migration Image Weight: Up to 1.3MB Lighter - PageSpeed Matters</a></p>
Frequently asked questions
Does moving to Shopify make images lighter?
In our data, yes. Median homepage image weight fell after every move to Shopify from another ecommerce platform, from 307 KB lighter for stores leaving WooCommerce to 1,330 KB lighter for stores leaving BigCommerce.
Does moving to Shopify make a site lighter overall?
Usually not. Across 1,246 stores moving from five ecommerce platforms, image weight fell a median 589 KB but JavaScript rose 773 KB, so total homepage weight grew a median 539 KB, and 57.2% of stores got heavier.
Why do Shopify stores get heavier after a migration?
JavaScript. 81.3% of stores shipped more JavaScript after the move. Top 100K stores added a median 963 KB of JavaScript, for a total increase of 1,239 KB.
The image savings from a Shopify move often come with the new theme, but keeping app and theme JavaScript in check takes a plan before launch. Need help with this? See our Shopify migration services.