RS_ is correct. Bankroll is defined as the total amount of money you have which you've earmarked for gambling -- meaning that if you lose it, you're done gambling forever until you make more money to replace it (which isn't used for other life expenses).

If you have a big bankroll, it is very difficult to be up 25% of it.

For example, if you have a bankroll of $300,000, you would have to win $75k in one session to be up 25% of it.

However, take the working class guy who lives mostly paycheck to paycheck, and brings $200 to the casino, which represents his entire gambling bankroll.

It would be very easy for him to win $50, which would be 25% in this case.

That's the one advantage of playing with a very small bankroll. If you bust it, it's rarely devastating and fairly easy to replace.