They only have 1 big in Bam.
Once Rob Williams returns, BOS has a big advantage with their big men with both Rob & Horford against Bam.
Dedmon is their next big. Theis is better than Dedmon. Nevermind Rob Williams.
It is not just their lack of size at center but also their lack of size on the perimeter.
They are small at forward. Jimmy Butler is the next biggest player next to Bam at 6-7 with short arms. PJ Tucker at 6-5 240lbs is their other forward. The rest of their perimeter players are guards. Herro is an undersized SG. Lowry is a small PG.
They have little size & length on the perimeter to help Bam in the paint. Bam is all alone on that team. Ready to be exploited.
Celtics with Jaylen and Tatum are too big for them on the wing. Jaylen is about the same size at Jimmy. They have nobody Tatum's size. G Will is similar to PJ Tucker. BOS two main guards in Smart and D White are both big PGs / combo guards.
BOS size on the perimeter will show up on the perimeter defense in BOS favour. They will be better able to shrink the floor, play the passing lanes and create turnovers. It is less about the offensive advantage but the defensive advantage. And defense leading to easy offense that way.
MIA is overmatched. This is not a strong Heat team outside of their 4 main guys. Bam has no help in the paint. They lack size at forward and on the wing. BOS is a really bad matchup for MIA.