It's because you probably 'forgot' how to sing properly. Most of the people who do not rely on knowledge, but pure talent often have to face this problem if they stop singing for a while. If you sang well without truly knowing how you did it (you did it on intuition), you will have trouble figuring out how you did it before.
The way you sing have much more effect on stamina than your physical shape. Lots of singers (Karen Carpenter, Elvis Presley, Johnny Cash, Freddie Mercury...) could still sing well in poor health and then die shortly after. By this I have not said that your health isn't important.
Smoking will give your voice a less clean (not necessarily bad) sound overtime, decrease vocal range (bad), but will give your vocals a darker timbre. Nat King Cole used to smoke before singing, for it would make his sound thicker and richer. Nat King Cole died of lung cancer because of this.
You can check this by asking yourself: Do I have less stamina for speeches (long conversations, presentations, etc.)? If you still have the same stamina for speaking as you did before, it is probably not your shape that has been affecting your singing. If this is the case, you should go back to the singing basics, because singing (in your own voice) should not be tiring, but just as easy (by 'easy' I mean natural) as speaking. This of course does not apply if you do not sing in your own voice, but imitate other voices.
If you want, you can send me a PM with a recording of your vocals.