Athelstaine
Explorer
If your animal companion is medium size, then a small sized character should be able to use it as a mount.
But-- lets be honest, those who made D&D 5E intentionally made wolfs the only actual possible real choice as an animal companion as anythlng else would be comparatively worthless and a disadvantage.
So by all means ride your wolf companion as it was the only animal you were meant to choose.
I guess it depends on a character's concept. I have a 12th level Forest Gnome Beastmaster/Ranger with a Giant Frog as a companion/mount.