This is a good example of how frame charts don't really tell the whole story. On paper, my large Roam has the shortest stand-over (30.7") of any of my "main" bikes, the others being a 20" Trek Verve (31.1" stand over) and a 21" Trek 750 (30.9" stand over). In practice, I seem to have the most stand over clearance on my 750, despite it being the "tallest" bike on paper. My Verve feels a little "closer", but the Roam feels the tallest. Of course, swapping the 38mm tires for 50mm tires only adds to the height (6mm nominal increase). I agree with you...Giants do seem to "feel big" for their size.
My local Giant dealer has my identical bike in stock, but in a medium size (Roam 2, Satin Grey color). Half of me wants to go check it out, but I haven't yet. The other half of me wants to sell the Roam and buy something like a Raleigh Tokul 2 (unbeatable value in a MTB). The third half of me (I said I was torn!) wants to keep my Roam and consider 27.5" wheels and tires. I feel that it's too tall as it is...I just have to decide the best approach from here.
There are worse choices to have to make in life, right? 