If heat is an concern, you'd probably want to spec a thicker gauge wire.
Yes, but (especially when Cu and Al are pricy) being able to use a thinner gauge can be a big benefit.
Regardless of wire gauge, bundles of wire noticably reduce your heat dissipation ability and therefore reduce your safe current carrying capacity accordingly. That's why electrical code significantly de-rates the current capacity of circuits which run more than just a few conductors in a single conduit or through the same hole.