A larger hole gives increase in lever travel.
The first seal, main pressure one, has to pass fully the reservoir hole to then build hydraulic effort in the outgoing line, any further travel to achieve this gives longer lever stroke before the caliper is moved. Also dependent on how far the primary seal clears that hole at rest as that distance is redundant before the hole is covered.
After the primary seal has passed, the hole then is open to the piston bore (non pressure) with the secondary seal clearing that void as the piston moves further. Without that second seal it's hard to stop fluid leaking out the back end of mastercylinder. It's this second action you see push fluid into the reservoir if you squeeze it with cap off as it clears low pressure fluid from that bore.
Bigger hole would likely wear the primary seal more as it doesn't rotate but always pass linearly over that port.