all axel sizes are the same...the difference is in the Outer Diameter (OD) of the bearing. Obviously the size of the hole in the mechbox shell has to match.
Bearings are made up of itty bitty parts. 6mm bearings have smaller parts than 7mm bearings etc.etc.etc.. The larger the parts, the more durable they tend to be under heavy load.
6mm was seen as "fragile...and not durable enough unless you're doing a relatively low power high speed setup".
7mm was seen as the "min" size if you're going to use them
Then 8's, 9's and now 10's (I think, I haven't used them) came out relatively quickly.
I've run some pretty stiff setups in 8/9mm's...no issues with the bearings holding up. Gears/teeth/pistons going...but the bearings still look ok.
That said...there's good bearings...and shitty bearings. When you buy cheap guns...they skimp on these types of parts...after all describing it as having "8mm Bearings" sounds great doesn't it?