I seem to be experiencing this on T-Mobile as well. I sent a text from my wife's iPhone to both my Google voice number and my T-Mobile number, and the image received by the Google voice number was significantly larger than the T-Mobile one. Is something happening at the carrier level?
Update: I believe I've narrowest this down to an issue with Google messenger. When I set hangouts as my SMS client, images download at a higher resolution and image size. So, it appears to me at least that Messenger is downsizing, and that this is not occurring at the carrier level.
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