so my brother is getting a bmx for x-mas, a pretty good one to(s & m frame etc), got it at a garage sale cheap
but the problem is, it has the bigger crank in the front, while he has mentioned that he likes the look of the smaller crank cog in the front.
now i dont know much about bmx bikes, so is this just purely cosmetics, or is there actually a difference between the sizes? and what and how much would it cost to replace these parts. im assuming i have to replace the rear cog to even out the ratio size?
If it's running a standard 44-16 gearing, there's a good chance that it has a thread on freewheel. Freewheels smaller than 16 teeth aren't known for holding up very well. The micro gear look that a lot of freestylers run these days is accomplished with a cassette style rear hub, and cassette hubs are expensive. You can't change just the sprocket, because it'll screw up the gearing. It'd be like a mountain bike, permanently stuck in the easiest gear.
If you're lucky, and it already has a cassette hub, and just needs a driver and sprocket. How the cog/driver comes off is going to depend on the brand of hub, but will most likely require a lock ring tool and possibly a chain whip.
The only real functional reason for the smaller gears is to help keep the sprocket and chain out of the way on some tricks. It's also a little lighter, but not enough to get the racers to switch over. Small gears are mostly a freestyle thing. There's some debate over different gear sizes giving more torque, even when at similar ratios. The fact is that larger gears are more efficient because the chain doesn't have to bend as far, but it's a really minute difference that won't effect any but the smallest riders.
One other thing to watch out for is the brakes. If they are mounted on the bottom tube, (chainstay) the chain will hit if you change to 25-9 gearing. If it is on the top one, (seatstay) you would be fine. You would need to change the hub and possibly the cranks, some will work some won't.