At 13 weeks my boys were getting unlimited Timothy hay, 1/2 cup of pellets, and 1 cup of greens. They were also around 2.5 pounds. Bombur has always had slight tummy issues, so I’ve always had to monitor his diet and adjust every so often. 1.5 pounds isn’t super unusual because she is a dwarf bunny and won’t end up being more than 4ish pounds full grown. You could try increasing from 1/4 cup to 1/3 cup to make sure she’s getting enough nutrients in. You might want to transition off the Purina pellets and onto a good quality alfalfa pellets for young buns, something like Oxbow or Burgess. Purina isn’t the best quality pellet, and is geared more for adult buns. Hope that helps

Babies need unlimited pellets ( unless a vet recommends to cut back due to weight gain )
Since she is skinnier looking and the current pellets gave her runny butt I would look at changing pellets
I had a similar issue with my boy and I changed him to burgess pellets and have had no issues since
Burgess is also one of the rated top foods for its fibre content and ingredients
If you decide to change pellets though you have to do it slowly (75% old food 25% new food for 2 weeks , then 50% 50%,then 25% 75%
Too sudden will cause issues for her tummy

I limited my boys pellets from 4months ish due to weight gain but sometimes bunnies need unlimited till 6 months …

My boy is eleven months now and so he’s limited to 1/4cup in the morning and then 1/8cup at night with his greens – some use slightly less and some slightly more – you will get a feel for amounts once you reach that and see how she is weight wise etc

I would def look to swap to a alfalfa pellet made for young rabbits from one of the top rated brands for nutrition though , oxbow or burgess where what I narrowed that down too from ingredients ( oxbow was best for my past bunny but it didn’t help enough with poppy butt for my current one so he is on burgess instead )

Because she’s switching right off the bat without mixing them in with old pellet, watch for any signs of GI discomfort. Not eating or pooping for 12-24 hours and any diarrhea.