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. There is no question that they forget such behaviors as how to find the door to go outside. The neurons in the brain diminish in number and vitality and older pets often will simply lose interest in the things that used to spark activity.