just think of it this way (as an imagery)

there's ur big ring (42t chainring) on ur front and a little ring (16t cog) on ur wheel. Every time u make a full pedal rotation on ur big ring, ur little ring rotates 2.625 circles. Get?

now if ur big ring is bigger (imagine something dramatic), then every time u make a full pedal rotation, ur little ring will rotate a lot more circles. Thus, u can pedal at the same cadence as before but go much faster. But at the same time, u'll also need more strength to be able to push down that pedal.

And vice versa, smaller chainring, easier to pedal, but u go slower at the same cadence.