A 34D has less cup volume than a 38D. The letter D is not a fixed cup size. It describes a cup step on a particular band. When the band becomes larger, the cup made for the same letter also becomes larger.
The number and letter work together
The band number describes the frame of the bra. The cup letter describes a size step relative to that frame. You cannot separate the two and still know the cup volume.
This means a 34D and 38D share the same letter but not the same capacity. The 38D is built on a larger band and has larger cups.
Use sister sizes to compare volume
Sister sizes make the relationship easier to see. In a UK-style progression, a 34D is in a similar cup-volume family to 36C and 38B.
34D -> 36C -> 38BA 38D sits two cup-volume steps above 38B on the same band. It therefore has noticeably more cup capacity than 34D.
Compare the volume family for your size
Enter the band, cup and sizing system to see the closest related sizes.
Find Sister SizesWhy the two bras may also look different in shape
Cup volume is only part of the visual difference. A larger band can change wire width, cup placement, strap position and the distance between the cups. The 38D may look broader as well as deeper.
Two brands can also distribute similar volume differently. One cup may be wide and shallow, while another is narrower and more projected. The label does not describe all of that geometry.
A D cup does not describe how a body looks
People often use cup letters as visual categories, but a D cup can look modest on a smaller band and fuller on a larger band. Body proportions, breast shape and bra construction also affect appearance.
The label should be used to select a garment, not to predict a person’s body from the letter alone.
The useful conclusion
A 34D looks smaller than a 38D because it is smaller in cup volume. Read the full size, check the sizing system and use sister sizes when you need to compare capacity across bands.