M'kay...It's like this: If you add teeth to the rear you increase the leverage that gear has and improve acceleration, like putting 3.73s in your musclecar over 2.73s.
Conversely, dropping teeth on the rear will, lower rpm in gears, slow the bike's acceleration somewhat, make it more relaxed on the highway and possibly, increase top speed.
It's about the RATIO (Warning! Grade 7 math content ahead! )
betweent the front and rear sprockets. Let's just pick two ratios, 1:3 and 1:5. Which is going to have greater muscle power and leverage? 1:5. The same applies to bike sprockets. The bigger the difference small (front) to big (rear) the greater the jump in leverage, thus acceleration.
Something else to keep in mind is that changes to this will screw up your speedo's accuracy (if that word can be applied at all to bike instruments ) so, you'l need to recalibrate it, if you care. This will cost money, i.e. buying a yellow box or something like it.