Assuming that the sliding forces can be dealt with, the maximum velocity of a flat tappet (which partially determines how much valve-open area you can “fit” into a specified cam duration) is roughly proportionate to the tappet’s contact surface length (not its area).
The math:
VM = (tappet diameter - .040”) ÷ 114.6 (.040” is a safety margin to prevent edge contact)
Solving for common tappet diameters:
.842" = .00700" lift per degree (GM)
.875" = .00729" lift per degree (Ford)
.904" = .00754" lift per degree (Chrysler, AMC)
.921" = .00769" lift per degree (Olds diesel)
.970" = .00812" lift per degree (mushroom)
.990" = .00829" lift per degree (stock GMC L6)
1.220" = .0103" lift per degree (VW Type I 30mm replacement)

A .970" mushroom is about 7.7% greater than the Chrysler .904".

Important: a larger tappet has NO EFFECT on valve motion, it merely makes it possible to design into a new lobe.