1. Summon a slime with size 20.
2. Kill it.
3. Collect experience.
4. Use /xp query ..... points.
5. → You have something like 7 points.
6. Do the same with a phantom.
7. → You only have 5 XP points

I don't know if this is intended or won't fix, anyway it is inconsistent and I see this as a bug also because the mob strength also increases with its size.