@farmerjan pretty much nailed it with her post a while back.
Can't let them gorge on hay from the beginning. They will always be seeking that stuffed gut feeling.
I'd be limiting hay. And giving a good quality hay. As well as CONSISTENT feed twice a day. Weighed for 2% body weight.
1% to start.
Bottle calves are by far the most expensive to get to maturity. And making them look good requires a bit of constant care and feeding. Especially at that transition from milk to feed and hay. I want mine eating grain before they start taking much hay. I want em to know what a feed bunk is so they will eat that first and then go forage on some hay. Don't want em eating hay and fighting trying to get em on feed.
At this point, it'll just take time to for them to fill out. But will most likely have bigger bellies from here on out.
Edited to add
I'd have never used a protein tub for my babies. I like spending time with em and feeding em is a joy to me.
Honestly, I've never had a protein tub on the place. Never saw the need.