Unlike most that want the Slender look to their IMVU Avatars I applaud you for wanting to be different and represent all aspects of what a Human resembles. Kudos. In answer to your question, The best way for you to be able to build your avatar to the specifics you want is to you the scalers/resizers. There are many available on there for male and female. It is a matter of playing with the percentages to get the look you want. Now although keep in mind the more you put on to build your body the higher your base Kbs will go up prior to donning any clothes on. But doing it this way until someone creates a actual "Fluffy" (as my kid calls overweight people) will give you what you seek. To be able to have the fluffy look and wear whatever clothing you may like.

I looked in catty and have found what they call combo scalers. This creator has put in her Product page where you can play with the scaler to make what you want so you can know exactly what size to look for. Here is for Shoulders, Arms and Hands http://www.imvu.com/shop/product.php?products_id=30904108 and for Hips, Legs and Feet http://www.imvu.com/shop/product.php?products_id=30904108 Hopefully this will help you achieve what you are looking for.

Good Luck!

Es