Unless you put it on a scale and map out the DFC, "smooth" is a relative term.....and even if the curve is mapped, how much pre-load the limbs have will affect how the bow feels.
Lots of things can have an effect--limb design, limb length, limb material, etc. etc. etc. No one thing determines it all.
Lots of bows these days have a good bit of pre-load on the limbs. Carbon has an effect on this also, and of the few I've drawn only one (Turkey Creek) didn't start out very stiff. That was one bow--maybe the exception. A bow that has a lot of pre-load may feel heavier to draw.
There's a lot of great bows on the market, my favorite over-all being Chek-Mate. Of the ones I've scaled, they gain 2 to 3 lbs per inch--depending on bow style, length, and draw weight. My 66", 60@28 Crusader longbow gains 2.5# per inch. It's one that most folks guess low on the draw weight--very smooth.
Chad