Tier 2 and 3 do more damage than Tier 1.
Nexus towers don't do more damage since there are two of them attacking at a time, but should have like 3 times the resistances of other tiers.
This way if someone gets fed and pushes towers too early in the game, diving an underfed opponent at tier 2 and 3 can become a lot more dangerous(you don't have the damage/tankiness yet).
Also why are Nexus towers made out of paper? The last line of defence is honestly just not there. Try to defend them against a Baron push, it's just so hard.