For example the movement of food along the tract may be slowed which can result in constipation. Digestive enzymes often are not secreted in proper amounts to fully digest the food. This results in loose stools and a decrease in nutrient uptake. Take a look at the photo on the right and you will see an X-ray of a dog that is constipated. The enlarged colon is packed with firm stool and veterinary help was required to alleviate the problem.
Some pets do well on special diets and supplemental digestive enzymes.
You are right. Some pets actually gain total body mass from excessive fat storage. In general, though, the older pet will have less muscle tissue that it did as a youth due in part from less usage of those muscles. As well, bone density decreases due to multiple factors, one of which is lack of exercise. As the pet's body seems to shrink somewhat there usually is a change in posture, too, so overall appearance can surely be different from what you recall of your pet years ago.