BMX Sprockets typically join onto the bmx cranks and come in various sizes based on how many teeth they have that the chain will sit onto. The higher the tooth count, the larger the sprocket diameter will be and the harder it will make it to pedal away but will give higher top speeds for example.  Some bikes come with cassette hubs and they range from 8 tooth rear cogs up to 13 tooth, and this is known as Micro drive, in this case you can run many smaller front sprockets to save weight and prevent damage whilst pedaling all the same.
Whereas some rear wheels still use a screw-on freewheel system that starts from 13 tooth to 18 tooth and will need much larger front sprockets to ensure the pedaling ration remains correct. Many people buy smaller front sprockets without knowing this and end up putting the bicycles into super easy pedaling mode because the rear still has a large freewheel and the ratio is thrown off because of this.

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