flat chest = bad at making/storing milk
larger chest = more milk storage capablities, etc
So, yes, larger breasts do generally indicate better abilities to rear young. It would be evolutionarily beneficial to prefer bigger boobs over smaller boobs. Instincts are usually pretty simple like this.
However, the extent that people in western society obsess over naked breasts might be enhanced over that of some tribe where they are constantly exposed to them. I imagine they would still find them arousing and prefer bigger ones, however... much in the same way that western males still think good legs are hot, even though it's common to see naked ones.