There are 3-5 Meter titans, 7 Meter Titans, 15 Meter Titans, and etc... :3
And that huge guy is the Colossal Titan (So technically it is a titan :p) and it can't destroy the wall at all. It can only destroy the gate in the wall in order for other titans to get in the town.
I think he knows they're all called titans but thinks that the smaller ones shouldn't be.
I read that there is a good reason for why the titans do what they do from the manga, didn't spoil myself as to what it is so I'm excited and hope it makes it into the anime. Without that the show would lack depth imo - especially when they explained in the latest episode how they only target humans. If there was no explanation as to why it would just feel like they were made-up villains to move the plot forward.