It doesn't necessarily make a cam 'bigger' - the lobe profile and design of the cam determines that. What it usually does is increase the valve lift velocity in the mid-lift areas of the profile (the ramps) and deliver more area under the curve for a given velocity.
For example here are the velocity numbers for my favorite two Potvin grinds - can't really show area under the curve in 'text'
:
a)
Potvin 3/8: maximum velocity of .008512 per degree of rotation happens at .1239 of intake ramp lift.
b)
Potvin 425: maximum velocity of .008519 per degree of rotation happens at .1925 of intake ramp lift.
Now, the
Isky 404-A: maximum velocity of .009738 per degree of rotation happens at .163 of intake ramp lift.
Roller Lifters: I'm not going to dispute what Barney said - as I would have to know the context of his comments (cam, spring pressure, etc). What I will say is that roller cams tend to take more spring pressure to keep them tracking on the lobes . . . and "rolling/spinning" is a part of that. If roller cams didn't work, you wouldn't see them being used in just about every top-competition engine today.
What is difficult with roller lifter profiles in flatheads is that we do not have the added benefit of a rocker arm and associated ratio and our base circles are very small. About the only way we can achieve high ramp velocities (to say match a radius) and increase the area under the curve is by using inverse radius/hollow flank profiles. You've probably seen a few cams like this - the lobes look more like "peanut shapes". A great example of this is the top race cams that Harley used on the KR flathead and also what Kenny Kloth had made for his record setting engines for Bonneville.
One of the problems/issues with inverse radius cam profiles is that it takes SMALL cam grinding wheels to make the inverse profile with the small base circles of a stock flathead cam blank - and most of the cam grinders use bigger grinding wheels.
Solution: If you bore out the cam bores of the block, use larger journals/bearings and increase the base circle diameter of a custom steel billet core, then we have a LOT more to work with. This is what most of the high-end Bonneville guys are doing - it is the only way to get the profiles and lift that they need.
Enough of my babble . . .
Dale