I feel weaker simply because I never use sumo stance, so my body's not used to it. If my body's not used to it, I'm less confident about the lift, and mentality is a huge element to the strength that you feel when performing a lift.
I'd have a hard time believing that there's a serious difference in the amount of WORK you can get done in sumo stance vs. regular stance, but as miked pointed out, when you lower your ROM, you lower the amount of work done. Less work requires less strength/energy -- or, since you're reducing the distance, you can increase the force and mathematically be doing the same amount of work.