Some master cylinders have bore sizes cast on the housing. One that is marked 19 / 25 is a stepped bore, 19 mm / 25 mm .

Stepped bores ( along with some internal valving ) are usually used to provide high volume / low pressure to bring caliper pistons out rapidly then switch over to high pressure / low volume for actual braking. If you had a large bore only, the peddle would be very hard, if small bore only peddle travel would be very long. This system is sometimes used on industrial equipment ( like forklifts ) to provide power braking without any " power unit ".