Well, actually, they're both "relatively small", since more folk are not Christians than are, and same for Islaam.
When taken together they actually do constitute a majority of people at 54.7%, but even if you take them as individual groups, they aren't small. One in three people for christianity or one in four for islam are not small minorities.
And this is taking a "Not christians and not muslims are a monolothic group" approach. The fact is that there is no other religious group that even comes close to the adherence of either christianity ofrIslam. Currently hinduism comes in at a distant third with 500 million less adherents than islam and approximately 1.1 billion less than christianity, and this is the only one that manages to make 50% adherence of either christianity or islam.
Buddhism is much farther behind, and has around four times less adherents that christianity and a third that of islam, and Taoism and chinto have something like 10-20x less adherents. And those are the only ones that have more than 100 million adherents, since when you go farther down then that it gets a bit ridiculous.
If we're speaking in "relative" terms then christianity and islam are most definitely not small groups, especially if taken together. And in raw numbers, they number in the billions.