Ya what Heather said is right. Addition to that try to feed always both the side.
Wearing sports bras helpd along with what the others said x
It is common. Mine had about 2 cup sizes between them so I couldn’t wear a normal bra either and lived in sports bras for a while 😂 I started to feed off the small boob first and then have a shorter feed off the ‘big one’ afterwards. Took a couple of weeks but they did even out - still a slight difference but no where near as noticeable. Always make sure you are offering both boobs for each feed too
You might need to try and feed and breast pump more on one side than the other to get them equal. Everyone has a more dominant boob that baby prefers more than the other. I would try to help the other by pushing it more so it will increase.