If a rune knight uses giants might but can't grow large, do they still gain the other benefits from giant's might ? strength advantage & bonus damage?

• If you are smaller than Large, you become Large, along with anything you are wearing. If you lack the room to become Large, your size doesn't change.

• You have advantage on Strength checks and Strength saving throws.

• Once on each of your turns, one of your attacks with a weapon or an unarmed strike can deal an extra 1d6 damage to a target on a hit.

Since they're separate bullet points and don't reference being Large then, yeah, you receive the other benefits even if you can't grow in size.

Yeah, your size doesn't change but it doesn't say the other effects do not happen. 

So I guess you would roleplay it as your muscles bulking up without your size increasing.