Its been discussed in other threads here in the forum.
In some ways it depends on the resolution of the sender's camera. Someone with
a older phone or feature phone may only be sending small pictures.
Most of the ones I receive are coming in at 720x1280 whether they are screen shots or camera shots.
(Tap the pic, open in Gallery, then tap the pic in gallery to see details about resolution).
You can also set the maximum mms message size on the HOX, and pictures are sized down
to fit. If your senders set theirs at very low sizes you get very small pictures. I set mine at
300k.