Anyone had any luck figuring this out? My girlfriend and I are on iOS 14.3 (gf has iPhone 11 Pro, I have iPhone 12 Pro Max). When she sends me images in Messages, even when we're both in the house and on wifi, the images sometimes get shrunk as soon as they leave her phone (I say when "they leave her phone" because my MacBook Pro and iPad Pro both receive the smaller versions of the images as well).
I also just noticed that on her phone, a test image we're using starts out as a 28KB jpeg, but lands on my devices as a 4KB HEIC. Seems like this could point to where the problem lies. HEIC is capable of storing images at higher quality and fewer bytes, but perhaps there is a bug in the conversion process that only manifests from time to time. Would be nice if there were any way to disable this conversion, if even just temporarily...