In my experience, when a couple moves in together (ideally on the way to getting married), their thiccness level slowly converges until they reach an equilibrium on being thicc in the britches.

Which can be good or bad. I’ve seen friends get healthier and gain unwanted weight. I’m just a neutral observer. Some people need more meat on their bones. Some people need to hike every weekend instead of drinking a 30 rack watching sports or playing on the computer or whatever. Ideally, a couple adopts each other’s best qualities.