We all know that tigers are found only in Asia and not in Africa. So is it correct to say that tigers are "more common" in Asia than Africa?
If you used 'more common', I would take it to mean that tigers occur with greater frequency in Asia than Africa, but that they do also exist in Africa in some capacity. The 'more' in 'more common' is the difference between the two things being compared - more completely I would understand the sentence as:
While tigers are common in Africa, they are more common in Asia
Since tigers don't live naturally in Africa (it wouldn't surprise me if some where there as pets), 'more common' is probably not the correct phrasing. Generally people would understand your point, in fact many probably incorrectly assume that tigers do exist in Africa anyway, but I would avoid using that phrasing.
Since tigers are not found at all in Africa, it is erroneous to say that tigers are "more common" in Asia than Africa.