My DD1 - who is now 2.75yo - has (had?) this. She has a sacral pit with a skin tag. We noticed it when she was 6ish wks old. I mentioned it at the next appt when she was 2m old. We got in for an u/s of her spine when she was around 4m old. She was super wiggly so the tech got an u/s of her entire abdomen practically. In doing so, she found the differently sized kidneys. On of which is 2 standard deviations bigger than the other. We were referred to a peds urologist. We debated going and actually canceled the first appt. Then we rescheduled. Due to long wait times, she didn't get seen until she was 10m old. The urologist talked to us and said since she was thriving and wasn't experiencing any issues of the urinary system (UTIs, etc) that he didn't want/need to do any testing. He had us schedule a routine 1yr follow up appt. We didn't go. She has never had a UTI or anything. She is thriving and from what we can tell there is nothing to be worried about. If she decides to every play contact sports, we might take her to a urologist and have an u/s to see what's going on (if anything still).
If you're worried, by all means follow up. If you have no reason to suspect there is an issue, then don't drive yourself crazy about it and don't feel guilty if you don't pursue testing.