First you need to be clear about what you're talking about.
In regular Bike-speak, sprockets go at the rear, and chainrings/chainwheels go at the front.
In BMX-speak, a sprocket goes at the front, and a driver goes at the rear.
If you're speaking BMX, the most obvious difference is the size, the tooth count.
For the same size driver, a bigger sprocket will make the bike slower off the start, but give it a higher top speed.
For the same size driver, a smaller sprocket will make the bike quicker off the start, but give it a lower top speed.
If you change the driver to match, start and top speed will remain the same, even with a smaller/bigger sprocket. A small sprocket will increase the ground clearance. Better if you're riding ramps as there's less risk of the sprocket/chain hitting the lip as you drop in. If you're not riding ramps, well, a smaller sprocket is still lighter, which is a kinda-sorta advantage. The downside is that the smaller they get, the faster they wear. And the chain wears too.
If you move from a big chainwheel to a smaller chainwheel by the pedals, pushing the pedals will get easier. If you move from a big sprocket to a smaller sprocket by the rear wheel, pushing the pedals will get heavier.
Look closly at the sprocket and you will see a small dot or indentation,you align both dots (cam and crank sprocket dots)
Big chainwheel(by the pedals) and small sprocket(at the rear wheel) = big effort. Small chainwheel and big sprocket = small effort
Normally that indicates a worn out sprocket. Replace and you should be OK.
Larrge at the front and small at the rear
well if you have a 8 tooth rear driver you will need a 23t sprocket, if u have a 9 tooth rear driver you will need a 25t sprocket, if u have a 10 tooth rear driver you will need a 28t sprocket, and if you have a 11 tooth rear driver you will need a 30t sprocket
remove bar, chain there should be a small cover over the drive sprocket, held on woth a screw, remove cover, then there should be a screw or bolt holding the sprocket on, remove that, pull old sprocket off, replace it in reverse.
As far a the sprocket in the front goes, i think they make them as small as 22 tooth, and in the back, it is called a freewheel or a driver. Freewheels are generally larger and drivers are smaller. Drivers can go as small as 8 teeth i beleave.
Depends on if you're looking at the wheel or if you're looking at the pedals. 1st gear, the one where pedalling is easiest, is the biggest sprocket at the rear, and the smallest chainwheel at the front.
The TDC "markings" are actually just a small hole in the sprocket(s) that align with a small hole in the head. Typically, use a corresponding drill bit to hold the sprocket aligned with the head during assembly.
a large sprocket
Small sprockets reduce the weight of your bike so that's always a plus. Also a smaller sprocket doesnt get in the way and they look cooler :)