Higher cc engine=more power, lower cc=less power, why .15 engine is slower than a say .18 or a .21. So if you have a .25 in hte savvy and bump up to a .28 there will be increase in cc thus incease in power output.
The relationship isn't quite as linear as that, there's much more to making power than just displacement. There are plenty of race-oriented .21s that are more powerful than some .25s or .28s, and there are .12s that are stronger than .15s and .18s.
That said, MOST .28-and-larger engines will put out more power than your .25.