Road bike/MTB fitting issues

Hello, new here but been lurking for a little while. Saw that everyone seems to be really friendly and helpful, so I thought that I would post my issue and see if I could get any advice.

My problem is this: I've been mtbing for about 2 months now. I have a 15" Specialized Myka HT that I love the heck out of, and I always thought that it was super comfy for me. Well, when the cycling bug bit me, it got me good, and I just went out and bought a Specialized Sirrus Sport quasi road bike to ride around during the week since I don't get to go to the trails more than about once a week. The Sirrus is roughly the same size as the Myka, even taking into consideration the difference in geometry, and I've been riding it a little bit more lately. Anyway, I hopped on the Myka today to ride to class and it felt awful and tiny and like I was squished up and bending in half. Anyway, the question is this: Is this normal and I just need to adjust to the difference in a road bike vs. mtb? Or Should I make some adjustments like raising the seat on the mtb? I don't want to mess with the road bike because it was professionally fitted, but is there something I can do to the mtb?

Sorry for being so wordy. Any responses will be greatly appreciated. Thanks!

Were you fitted to the mountain bike?
How tall are you? 15" seems like a small size.

I was fitted for the mtb, actually. I'm about 5'7" but I have weird issues with torso to leg ratio and so the smaller bike was much better than the 17" they originally had me on.