Which is bigger - a lion or a tiger?

It depends on the type of the tiger. A lion is bigger than an asian tiger, but not a siberian tiger.

A tiger is 10 ft (305 cm) on average and 550 lb (250 kilo)

A lion on average is 9 ft (275) and 420 lb (190 Kilo)

so a tiger is slighty bigger, the largest lion ever was 825 lb (375 kilo) while the largest tiger was 930 lb (422 kilo)
Tigers are bigger than lions if you mean tigers.