Nice looking fish-doesn't actually look too skinny for a planiceps.
The last one I saw in the flesh was some 25 years ago when I work at my LFS when at school-never seen one since. It was in a display tank and found that no matter how much you fed it, it never looked fat (just like you said). I've seen photos of massive ones though.
They can look fat, just not to the same degree that say a TSN does. My 16" one ate a 4" perch the other night and it looked like it swallowed a golfball. Incredibly cool shovelnose.