Jack's Fat

So he needs to shift some lard. However he's never been a big eater, yet last night I was looking at him and I think he's starting to resemble a barrel on legs.

This morning when I went to give him some breakfast I gave him half his normal amount (going on the theory less food and more exercise) but he just laid in his bed and didnt bother until I said 'Come on have your breakfast' and he did, but begrudingly. Casper wolfed his down like normal and sat back waiting for Jack's scraps

This is the first time in 5 years that Jack has gotten a bit podgy, his diet hasnt changed that much but I can figure out why he's put more weight on.

Can anyone think of a few health reasons where a dog would put on weight but still be quite healthy and happy in himself ? I know he's definately not getting more food as he wont eat much anyway.

Thanks
xw

he cud have developed a small heart mumour . if he was female id hav said a thyroid prob but in these cases ur beta to seek vet advice and rectify prob sooner rather than l8r