There are two main types of BMX bikes. Race and freestyle. Tiny cogs are used on modern freestyle bikes to help keep the chainring out of harm's way. Many stunts will bend up your chainrings if they're big.

Back in the day 14T was the smallest rear cog available so the chainrings had to stay big to give you a rideable gear. Now with 9 and 10 T rear cogs you can go tiny up front and still have a decent gear.

Racing bikes still have bigger chainrings and freewheels because they're smoother and wear longer.

Common Race gears on 20" BMXers: (FRONTxBACK)
36x14, 41x15, 44x16, etc.

Common Freestyle gearings:
30x10, 25x9, etc

BMX bikes are typically shooting for somewhere around 52 gear inches.

If you really wanna go places without spinning like a hamster on crack, shoot for around 72 gear inches.

You can enter cogs and tire size here to figure gear inches. I use a custom 1555 mm wheel diameter to approximate 2.0" BMX tires. http://gear-calculator.com/#