They say eating lots of veg to prevent constipation. On the other hand eating too much tends to retain inside the bowel? I'm confused.

It Depends. It depends on several factors, like which vegetables you're eating? Also, you must make sure you're drinking plenty of water, maybe more than you're currently drinking. It's usually not a good idea to alter or make major changes in one's diet all at once. It takes a while for one's body to adapt. So, maybe try and just make one change, see how that goes and after a while, change something else.
Excellent point. The same thing happens when we give metamucil or psyllium husk to patients! it can act as a tonic, its not a laxative, so thats why im naming it a tonic or it can act as a bulking agent at higher doses. You need to find the balance that works best for your body. Add fruits, such as apples and pears with skin to your diet. That will help with regularity. Experiment and see what regimen works!