Fates literally do not scale.

[Jinko]

It seems like there is tons more monsters when more people join the fate then when it's just you and one or two others.... I'm not sure what you mean by not scaling? If you mean monster level, well of course not your char is scaled down to the fates level.

I think by scaling we expect more HP/defence and not more monsters, at least that's the obvious thing which I would think of.

By throwing more monsters at a FATE you aren't really fixing things for slow casters classes, as much of the time things die before you even get a spell off, that's why people resort to instant cast AoE spamming to try and get some credit for participating.