Rachel (my 45 lb., long been out of diapers, 17 days away from being 5 years old) balked at doing this for me slightly but finally gave in and modeled the size Medium-XL. The lighting isn’t great but the fit is just fine!
And below is my 2 1/2 year old 26 lb. son Drew in the NB/Med. I had to bribe him with a candy cane to get these photos!
At first glance I thought there was no way he’d fit but it did fit with room to spare too! You can definitely tell the size difference between the NB-Med and the Med-XL though and when you’re holding the smaller of the two sizes in your hand, it’s easy to see how it would easily fit a 5 lb. newborn without too much bulk.
Of course, my son is super thin too so I cannot assure you this would work for most 2 1/2 year olds.
Both my kids have been using the potty for ages now and so I not longer keep diaper pins around. Regular safety pins did the trick though.
If Drew were still in diapers I think I would prefer the Med-XL size just because I can :o). But this is for the sake of modeling purposes.