If the bar/chain doesn't get enough lube, the bar gets worn away prematurely. It's aluminum, the chain is steel. Wear makes the circumference of the bar smaller, which is the same effect as making the chain longer.
I'm not saying this is what happened in your case, just answering the question of how it 'could' be the bar, not the chain.
You should be able to find a diagram which shows how the bar adjuster comes out. I would guess you simply unscrew it until the piece that engages the bar drops off the screw. Take it out and clean the full length of where that adjuster slides and see if that gives you some more travel on the bar.