Not finding DRK squishy at all.

[Reynhart]

Higher def does mean less damage, however stat formulas are not the same for 51+. Which should be obvious given Skill speed requirements are different for example.
The game has been rebalanced for the new level cap.

This should be the case only for mob above lvl50 too.
For example, if you want to take the same damage as a level 60 vs a lvl60 monster, you'd need a higher Defense/Attack ratio that you would at level 50 vs a lvl 50 monster.