So what's your suggestion then?
Make if more fragile but buff its damage?
Your arguments are fairly incoherent. The target size difference is small to almost negligible for shooting on the move. SOV and UKF have a 0.5 moving modifier, so the actual difference is effectively 1 target size.
Also the "small tiny M10" is actually not that small, I don't know where you get that from.
Mobility. Acceleration/max speed. Comparing 105mm and Brummbar, the 105mm has better mobility and a turret but worse AOE/survivability, so trade-offs are fine. However, given how brummbar can reach 312 armour with 800 hp and a mini nuke dispenser, it should be less mobile. That's all I think.
M10 tiny as in weak/low hp. The whole point of M10 is to be extremely mobile for hit and runs but due to it's slow turret rotation it falls flat hard, but that ain't the point of this thread.
So to sum up, I'd only nerf the acceleration/speed on the brummbar, and in return, buff rotation. That would be a buff for smaller modes and a nerf for larger (where it's more dominant).
Most 3v3+ maps don't care about having a turret or not since they are extremely lane-y and there is a massive lack of flanking pathways, especially given how 222 and pios have dominant sight for any flanking attempts.
EDIT: Shame I didn't put the replay where I did win a game, but it was sort of a Pyrrhic victory. A brummbar just drove up to double AT guns which were spread out for at least 25 range or so (on average we had better map control throughout the game), bounced 5 shots, took out both AT guns and snared outran the zook carrying echelons with low HP. Now that was bad RNG, sure, but it's not some rare RNG. I've had plenty of such scenarios against the brummbar. The only reason why it got out alive was the fact that it could outrun infantry with handheld AT weapons. Panther and P4s should outrun infantry even when snared, but a brummbar/105mm/ST/priest/... should not.