Hello @pinker thanks for reporting this. This looks a lot like an issue that our Team is working on, it affects images used as fills in symbols. The image looks ok in the Mac app but it renders at the original size in the Web app.
If this is your case, we’re close to releasing a fix and if you can drop us a line to productsupport@sketch.com we’ll notify you as soon as the fix is released.
If this doesn’t look like your case, could you let us know via email too and if possible, share a sample file for us to look at? Thanks in advance