When you use the birth control pill, patch, ring or shot, implant or IUD with hormone, it keeps the lining of your uterus from growing to the full thickness that it would require to support a pregnancy. This is one of the main mechanisms by which birth control works to prevent pregnancy. If the lining is thin, then there is nothing for the embryo to stick to.
If you usually bleed, 7 days, you can expect it to go down to about 5 days. If you usually use 5 pads, then it might go down to 3 pads, etc.
It is expected and normal for the bleeding to get lighter on the birth control pill, patch, ring, implant, injection, IUD with hormone.