Can a small character ride a huge animal?

[Zautos']

So can a gnome be a mammoth rider?

[Nefreet]

Your only restriction is that you won't be able to "fast mount/dismount", but that's a restriction built into the Ride skill.

Gnomes and Mammoths are just as viable as Humans and Dragons.

[DM_Blake]

And just as viable as Legolas and Olyphants.