Why do obese people tire quickly?
Fatigue on exercise is proportional to unmet demand for oxygen and glucose (sugar/fuel).

In people who are "fat":


They typically do less exercise. Therefore their cardiovascular system is not conditioned with the heart being able to cope with increased exercise i.e. it is unable to beat more efficiently and instead must beat A LOT more faster to cope consuming more oxygen and glucose. Even at rest, they will typically have a higher resting rate. They also have less efficient muscles (i.e. the muscles themselves can't breakdown energy as well, use it as well and the body isn't as great at picking the right set of muscles)
The fat itself can compress vessels carrying blood. This pressure increases blood pressure. This is worsened by a typical high diet of salt and again by typical lack of exercise. Higher blood pressure means the heart has to beat harder and faster to pump blood around the body
They weigh more so each movement requires A LOT more energy. Their resting metabolic rate is higher than a thinner person as is any exercise they do. 
On exercise they do not lose heat fast enough. This impairs the breakdown of glucose and also more energy is required to lose the excess heat
They may be subclinically diabetic. Their body may have lots of glucose floating around in the blood but they can't use it because their muscles can't absorb it.
They have far increased rates of build up of fatty plaques on the blood vessel walls. This narrows the vessels and only multiplies with all the other problems
Then there's lots of other more minor but very important reasons like psychological (reduced capacity to brave through fatigue), reduced sleep meaning a higher baseline of tiredness (in the extreme side caused by obstructive sleep apnoea), increased rates of smoking (which does millions of things to reduce exercise capacity)