It is relatively small, yes, compared to the onza t-raptor, which i think (correct me if i'm wrong) 15", also if you look at uci style frames they have as little as 4 or 5 inch seat-tubes. As for the frame, its a bit on the heavy side, but as your leaning this shouldn't be too much of a problem. 140 quid for the frame only is not cheap, have a good look around and i'm sure you will find it for less.
PS:
Don't worry though, that bike got ridden into the ground! plus the owner did a rather nice static gap to chainstay which helped speed up the snapping process.