Yes they get a bit smaller at the end to reduce the spread.

Bullets are quite malleable and it’s normal for them to deform a bit during firing. Bullets for non-shotguns typically get forced down a bit in the forcing cone before being deformed to fit the grooves in the rifling.

“Rifled” lead shotgun slugs have these grooves around the outside that allow them to deform down to a smaller diameter. You can shoot them through a choked shotgun barrel just fine.