Boy I've been busy with some experiments. I've been trying to see how fast and how slow the Mii Fighters are when I make the shortest and thinnest Mii, along with the tallest and fattest Mii.
Based on my research, it seems that the lightest possible Mii has up to 1.22x its normal ground mobility, while air speed was about 1.185x the normal value.
As for the heaviest possible Mii, it seems to have 0.82x its normal ground mobility, while air speed seems to be 0.85x the normal value.
Amusingly, the heaviest possible Mii Gunner ends up running slower than Mega Man's walk speed, while the lightest possible Mii Brawler has higher air speed than Limit Break Cloud, and almost rivals Zero Suit Samus's running speed.