Bikes made for drop bars typically have shorter top tubes than mountain bikes. This is because you spend a lot of your time stretched on the hoods or in the drops, where the forward curve of the bars gives you more room.
As a point of reference, I'm 6'3" with a 35" sleeve. A 24.5" mountain bike top tube feels fine, but the 23.3" top tube on my Gunnar Crosshairs feels too long--I have to use a stubby stem to make it work (although I like a bit more upright stance on the road bike, since I tend to ride the MTB more aggresively).
Eric