So a centaur counts as one size larger when determening its carrying capacity. But carrying capacity is strength score times 15. Size is not relevant in that calculation. Am I understanding something wrong?

Thx in advance,
Civer

https://www.dndbeyond.com/sources/phb/using-ability-scores#LiftingandCarrying

Size and Strength. Larger creatures can bear more weight, whereas Tiny creatures can carry less. For each size category above Medium, double the creature's carrying capacity and the amount it can push, drag, or lift. For a Tiny creature, halve these weights.

So a Centaur of size Medium is considered Large (for carrying capacity) and therefore can carry twice as much. Or Strength x30.

• PHB Chapter 7: Str x 15 is carrying capacity. Push/Drag/Lift is carrying capacity x 2. 
• Each size category above medium x2's that.
• Things that will increase size category:
  • Actually being that larger size (e.g. Enlarge/Reduce, Rune Knight 3 Giant Might class feature)
  • Having the Powerful Build racial feature (Bugbears, Goliaths, etc)
  • Having the Equine Build racial feature (Centaurs)
  • Having the Totem Warrior 6 Aspect of the Beast - Bear class feature

What they said. Gargantuan creatures are 8 times stronger than medium creatures per STR.