The milky sap tells me that your "fat plant" is almost certaily a Ficus probably F. microcarpa sometimes called F. retusa. The milky sap is latex. Privits do not produce latex. The leaves resemble privit but the two plants are not closely related at all being in different families. Their flowers are radically different but I will not go into the details of that here. Look them up in an encyclopedia of horticulture.

Hey there , I found out they are Ficus microcarpa . Even though they are very common now here in so cali , they are very very cool plants and seem to be fun to have around . I've bought a few and even though they can go into six inch pots , I potted them in ten inch pots and in full sun . I want bigger fatter roots . I'm not sure exactly how to go about getting them but I figured I'd just let them be for five or more years and grow them hard in the mean time ... Or do you have any advice ?

I answered your question on the other thread about Ficus roots. Basically you need to grow the top in order to get big roots.

Big fat roots are seen on Ficus ginseng, and there are lots and lots of them around the big box stores.

Larke , I've only seen a few Ficus benjamina ' ginseng ' available in the last two or three years here in cali . They've been replaced with Ficus microcarpa with the big fat roots . They are about $20 at the big box stores , I have about six of them that I have in big huge pots and I'm going to keep them in there for a good five to seven years and go from there .
jack_r , Are you sure I need to grow the top to get the big fat roots ? Wouldn't keeping the top short make the energy go back into the roots of the Ficus ?

bamboo, yes I'm sure that you need to grow the top in order to get big roots. Think about it. The roots are woody which means that they contain lots of cellulose. Cellulose is made from sugar and the sugar comes from the leaves where photosynthesis occurs. More leaves, more cellulose. Jack

To clarify that a little more 'top' equals 'vertical' growth. The more you allow a tree to go vertical, producing more branches and leaves, the heavier the surface roots will become. Since strong surface roots are a primary part of the design of any good tree/Bonsai; hence the reason most quality trees you see have spent a good amount of time in the ground or a large container prior to finish styling.

Bob