How's it doing at the base? It looks like there was rot that scabbed over? No? See the pic below. If it had/has rot, that's probably the issue.
To get them fatter, introduce them to more and more light. You'll probably want to wait till next Spring to do this. This makes a HUUUUGE difference. They'll never reach their potential without close to full sun.
Many Trichs will grow log-style naturally: candicans, brevispinulosus, spachianus, ect. Bridgesii are prone to falling over as well. They grow upright for a long time then one day an arm will just kind of start leaning. Over the course of about 24 hours or less it will be touching the ground. And, yes, you can just leave them on the ground and they'll root, then the tip will start growing upwards. It's like they crawl around if you give them the space.
Lack of direct sunlight is probably the issue unless: 1) it's a cross of a Trich that naturally falls over, 2) its rotting at the base.
I would cut it up and re-root each section.