Turn your bike upside down, or get someone to hold the rear wheel off the ground for you, and run it through all the gears. You'll see how shifting onto the biggest front chainring makes it hard to pedal, and shifting onto the smallest front chainring makes it really easy.

In the rear, it's the opposite, because when you change gears, you are actually changing the gear RATIO.

I just think it really helps to see what is happening when you change gears, and you can't see it when you are riding.