First, bike sizing, especially mountain bike sizing, is pretty much virtual now. You can get by just paying attention to the XS, S, M, L, XL, XXL designations.

But if you compared the measurements of your road bike to a serious mountain bike that fits you (no Trek 820s, etc.) you'd find the virtual top tube a lot longer, the seat tube (actual and virtual) a lot shorter, and the bottom bracket a lot higher. The bottom bracket height of most road bikes is 10.5 - 10.8 inches. If you were that low on a trail bike you'd be dragging the cranks over rocks all day, so bottom bracket height is 13-14 inches, before sag, depending on the discipline. Now imagine straddling a 55 cm frame with the bottom bracket 3-4 inches higher. Can you do it?

Now let's look at the components of reach. On a road bike they are the top tube, the stem, and the handlebar reach that goes beyond the stem. The top tube is short to keep the bike nimble and laterally stiff (and increase toe overlap ), and the stem tends to get long to compensate for the shortness of the top tube and help the rider weight the front wheel to improve high speed handling. On the mountain bike, you have the top tube and the stem. For the most part, the bar is behind the stem, especially on 29ers. The stem is short to improve low speed maneuverability. The top tube is long to maintain that athletic, forward leaning posture over the bike, but not so much as on a road bike. The athletic posture is still best for getting power to the rear wheel, steering, especially on descents, maintaining balance, and keeping both wheels on the ground while climbing.