It's not something I ask about, admittedly, but I've not heard of anyone have that problem before.
Regardless, sticking a little plank of wood, or a piece of or paving stone or deck plating across the base so that the raised foot has something to rest on would make the pose a lot more natural - it looks a little odd hovering like that.
Basing is also a great easy and subtle way to bring a little more completeness to the model - just so the characters aren't standing on a featureless plane.
Th Fire Warrior is clearly too small, especially the hips and head, but I don't think it's something that would bother me overmuch, especially if you put a normally-sized head on there somehow.
I think the height difference is made more obvious than it would otherwise be by the direct height comparisons - on a table, spread out as the characters naturally end up, I don't think it'd be such a problem.