Zentradi are 30-50 feet tall. Humans are...not. In order for Zentradi to interact with humans on a normal basis, they undergo a procedure called miclonizing/micronizing that reduces them to human size.
Normally, their appearance is unchanged between Zentran form and human form, but for some reason KK's appearance shifts from busty woman to washboard flat kid.
The procedure also works on humans and human/Zentran hybrids. For instance, Max Jenius' third-youngest daughter, Emilia, favors a full-size form.