That's not quite how the math works. Assuming there was a size/mass minimum for fossilization, and that we expected that to be the only qualifier for fossil occurrence, we'd just need to expect X% of species to exhibit sexual dimorphism depending on how frequently we see males&females of the same species.
For example, if you needed a bone to weigh 5kg/meter and males are 5kg/meter but females are 4.5kg/meter then you'd expect to see a very low number of females even though they're only 10% smaller.
Obviously none of this is the case, and this isn't the deciding factor on why it's happening. Just pointing out where you missed things slightly.
You're right, but that doesn't fit in a snarky one-liner.
If we were serious about this bone mass minimum, we'd need to consider why there are fossils of smaller animals, or big but hollow-boned animals. That's a rabbit hole with many hypothetical variables.
For example, if you needed a bone to weigh 5kg/meter and males are 5kg/meter but females are 4.5kg/meter then you'd expect to see a very low number of females even though they're only 10% smaller.
Obviously none of this is the case, and this isn't the deciding factor on why it's happening. Just pointing out where you missed things slightly.