But we aren't discussing "an individual" - we're discussing "an individual amount" - which is how "Individual" is defined for each Form. In this context (Corpus), a person no greater than Size +1.
And, yes, Giant Blooded magi are slightly penalized, because their "personal" spells need to be larger to affect them (by the RAW). A SG/Troupe could easily handwave that away, tho', and I sincerely doubt if anyone would care to suggest that such a ruling would unbalance any part of the game.
IRLS: i had a giant blooded magus, and it was a pain, because okay, i had "protection" against corpus basic spell, but no formulaic spell could work on my (except on group) and boosting characteristics Creo spells were at individual...
And as the players stated that the spell must be cast on the true size, even if muto-ed, i could not diminsh size to be cast healing spells EG...
Sta 6 and damage range are the avantage;.. but the latter could have been a good MuCO spell... so only Sta 6 is a benefit.
As for corpus spell , i had no chance to have it effective: enemis had "group" spells and other mentem spells, which have no size 
This sounds to like the GM slightly screwing you 
Isn't Group defined as a clearly distinguished collection of up to a dozen (IDHMBWM so it might be another amount) Individual sized targets? So the Giant blooded magus cannpt be affected by Group spells either, if they don't have an added magnitude for size. For Corpus that is, Mentem has no size modfiers IIRC.
PS
To the OP: You should really use a subject specific title for the thread, not just 'Quick Question'. No offense meant, but some users of the forum might not read such a post, and you might miss out on their input.
Group is the mass of 10 individuals (i think it's stated in the size box). So : 3 giant blooded magus and 1 normal, or 10 normal, or 100 size -3 etc. For mentem, as all minds have size 0, its 10 of them 
Ok, so the cap is simply the sum of the mass of a maximum of 10 Individuals? Right, I thought you had to look at the different individuals individually.