Any brake boost at all is better than none, but if the piston diameter is noticeably smaller boost will be reduced, as it is directly proportional to the piston diameter. When there are more square inches of surface area for atmospheric pressure to push on with a bigger piston, the thrust on the hydrovac piston is increased. Just an inch or two of diameter difference will make a huge difference in stopping effect.

Back in the late 1950s my father put a 3/4 ton truck hydrovac on a 49 Oldsmobile 98 sedan. That thing would stop on a dime, and give some change back!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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