indeed ..

Top offenders were essentially 2048 pixel wide png photos which should have been 1024 wide jpgs at max - weighing 6MB instead of 250k for a difference the human eye probably cant detect : )

but.. more than that, just an endless stream of js html css inclusions .. I suspect 85% of which is dormant, 20% actually repeated, if I were to guess.

hehe

I'd hazard it's mostly images and/or the plugins the store has installed to track conversions and / or extend the Shopify store. I do admit plain Shopify stores/themes are quick and lightweight, and the plugins are to blame, but I also point at Shopify for blocking some easy features to be supplanted by plugins.