If you actually read the manga at all you would have learned why that’s the case. You see, all the dragons in the series use a magical spell to take humanoid forms. This spell creates their forms under a simple rule: their form is decided based on how they perceive themselves. Take Lucoa, the blonde one with huge tits as an example. Her real name is Quetzalcoatl, a harvest deity in Mesoamerican folklore. The “Venus figurines”, a broad term describing statues/carvings of voluptuous women, are well known to the archeological community due to the fact that they’ve been found everywhere from Siberia to Europe. These figurines have long been thought to be a universal symbol of both fertility and good harvest. Keyword there is “harvest”, just like Quetzalcoatl. Thus, because off these two facts Lucoa’s voluptuous form was justifiably made.
The character you’re referring to, Ilulu, is a different story. For the sake of avoiding spoilers I’ll keep it vague, but her perception of herself was warped by tragic events in her past. She’s essentially mentally conflicted and that’s why she appears the way she does in her human form. Go read the manga or watch the upcoming anime to get the full picture.
Reading this changed my life.