With a bigger master cylinder, you need to press the pedal harder but a shorter distance than a smaller master cylinder for the same braking effect. A bigger master cylinder reduces free play from pedal off to the depression point where the brakes begin work.
Fitting a smaller master cylinder makes the brakes feel better as if you've fitted pads with more bite.
Personally, I think Holden increased master cylinder size to 27mm with Brembo's without realising the 25mm isn't on the cusp, it's massive overkill for the stock brake system. Probably the same bloke who stuffed up the Redline wheel widths and offsets!
That‘s very true with old style non boosted brake systems where the pedal effort strictly follows Archimedes geometric reasoning regarding pedal leverage and Pascal’s theory regarding the hydraulic pressure. That is, the effort you exert via your leg is what is translated through the pedal leverage to the hydraulic system and impacts the brake pedal feel in a very analog way without other things masking said pedal effort & feel. Its something even I can understand
But I’d have thought that in modern boosted system, especially where ABS plays in the mix by offering front to rear and side to side brake bias through a secondary control system that impacts line pressures, the brake vacuum booster would have a bigger impact on pedal feel and pedal effort than the master cylinder size (as would the calibration of the secondary system wrt ultimate brake behaviour). Modern stuff just seems more complicated with many more things in the mix
Whatever the case, the master cylinder must be dimensioned for the callipers fitted to the vehicle so one doesn’t run out of master cylinder piston travel during application. And if I’m not mistaken, it needs to cope with the fluid volume needed to move the pads from their rest position to rotor contact which is based on slave cylinder sizes and actual rest to contact distances they travel. And as has been said a few times, 25mm or 27mm are sufficient to do the job with Brembo‘s all around so I suspect it was marketing or engineer over exuberance to put 27mm MC on some variants.
The more interesting issue is what Holden did from a compliance perspective so as to verify the downgrade from 27mm to 25mm is ok (given that when one picks up a 27mm MC from spare parts they are almost certain to get the 25mm calliper). I’d have thought Holden would have needEd to lodge some paperwork with Department of Infrastructure, Transport, Regional Development, Communications, and the Arts when such critical parts are superseded and replaced with different parts, especially when the authorities make our life harder than it needs to be when it comes to brake mods