Dads that spend all their time lifting and building their body need to keep eating. They need to fuel themselves to make sure they have enough energy, calories that allow their muscles to bulk up. It's hard forcing themselves to eat in order to keep growing, but soon enough the habit sets in and they're unable to stop.
They find they can no longer control themselves from eating, their stomachs constantly empty craving food. It becomes a hassle, always trying to satiate their hunger and fill their tanks. They lose any ability to control what they eat, settling on cookies and sweets when nothing else is available. Slowly they clear out their house of everything edible.
It isn't long before their diet takes an impact on their body. While they keep their upper musculature with constant exercise, their midsections begin to wrap in lard. Any hope for abs they had is gone, their growing paunch replacing any existing definition. Every extra calorie goes straight to their belly, a pouch that grows softer and wider around their torso, jiggling and shifting with their movement.
They don't know why it's happening but don't seem to care. As long as their arms and chests are visible pressing against their shirts, the growing orb under it all doesn't matter. They can't stop it, becoming used to their expanding gut hanging out above their waist, a constant accompaniment for their toned muscles.