Of course they feel better when they are leaner, the extra fat only puts unnecessary stress on their joints. Cookie was at his best when he was 7, so a 6 years old I still consider to be a young dog. I am saddened when I see 4-5-6 year-olds, and even older Labs who look like fat sausages and can barely move. They look miserable and I can only imagine how they would feel in their senior years.
You did a great thing for them!