I think the main question is, does it start with 1 tail firdt at born and progress to the 9th tail later on. Or it was 9 tail since it born. Which the answer is the similar, the body it self build for it somewhat. Any animal first born dont have the strength to even move it body, but the law of the jungle demand animal to at least able to stand up then walk and run to defend it self. So a nine tail fox (either progresses from 1 to 9 tail or born eith nine tail) is demanded to be able to bear its body and limb and use it.
this ^ plus the centre of weight can be made back to chest area by increasing weight in front. A good method might be a gradiation in density of bones.
the progression for a nine-tailed fox is usually a certain level-up for fox spirits. So we can assume that only one tail has true physical form, and the others are just a very VERY compresed magical energy getting physical form but having no real weight since as an energy can be controlled via soul/mind/spirit