Well I dont know if one being more than twice the size of the other is exactly a minor difference, but yeah, ill agree with the mobility thing tho.
LOL, I know the difference between the two, I wasn't going to get into details as 99% of this site has never seen anything close to that in real life and won't know wtf we're talking about anyways.. if it makes you feel better I'll clarify, as in the pic you posted
the smaller 'mini-gun' is used on helicopters/naval warfare vessels, helicopters, etc. and larger 'cannon' is used on A10's, frigate's, destroyer's, etc.
Either way, the thing goes through 3000 rounds a minute, the amount of weight in rounds alone is enough that very few people could realistically arm themselves with one, not involving it being mounted on a vehicle of course