I'm tight in some 36C, looser in some, lost so much weight since the start of the year that breast growth has slowed down much since I'm losing a bit of fat from the breasts.
Some 36C have a tight band, others are looser, don't really get it since 36 should be 36!
The bigger the band, the looser you'll be in your bra even if your tit hits the top of the cup, if your breasts are not fully mature yet (fully rounded). Smaller bands are more forgiving since even if the cup is round, the side angles are more acute and will still sort of fit.