Quick answer
The measurable difference
Megalodon is larger in the conservative ScaleCheck comparison: 18 m versus 13 m, and about five times the displayed mass.
Why this matchup matters
Different bodies, not just different numbers
Both are portrayed as ultimate sea monsters, but one was a shark and the other a marine lizard with very different bodies and fossil evidence.
Human scale
How the gap feels
The five-metre gap equals almost three standing adults. Even the smaller Mosasaurus still spans more than seven reference-human heights.
Evidence boundary
What the comparison cannot prove
They were separated by roughly 43 million years. Size estimates for both rely on incomplete skeletons, so this is a comparison of representative reconstructions, not two measured specimens.