Full grown male bengal tiger vs full grown male african lion , who will in a fight and why?

It can go either way. This has been proved by the multiple fights staged between the two over the centuries. The biggest tigers are bigger than the biggest lions, but not all tigers are bigger than all lions - some lions are bigger than some tigers. There's also health, experience, aggression, age and so on to consider. If one was big but timid, and the other small but very aggressive, it's likely the smaller one would win, despite being smaller. It's not always about size. Lions tend to have more fighting experience, because they often clash with other prides, while tigers try to avoid conflict. The mane of the male also gives him added protection. These things often make up for the greater size and strength of the tiger, meaning you really can't say one will win all or most of the time - it depends entirely on the individual lion and tiger in the fight.