Scootaloo, now an adult and married to Rumble, is a very small mare. She gets mistaken for an adolescent constantly, but she deals with it. Meanwhile, Rumble is a HUGE stallion, easily a head and a half taller than your average guy. They decide to start a family, but to their surprise, Scootaloo winds up pregnant with quadruplet colts! Realizing the pregnancy would be extremely hard on Scootaloo (even moreso given the last four kids were all big foals) her doctor advises her to seek out magical assistance. Starlight Glimmer concocts a spell to transform Scootaloo’s body to be able to adequately carry the pregnancy to term, and over the next few months, Scootaloo grows bigger and stronger, the process speeding up as the pregnancy progresses

https://www.fimfiction.net/story/375542/little-mare-big-family