First, I agree with Mr. Loewen that a small party that becomes part of a coalition will tend to have an outsized proportion of power by virtue of keeping that majority alive.
However, by the same token, small parties that are not part of the governments in proportional representation end up essentially with no power. That can give rise to an incentive for strategic voting on the part of voters.
Again, suppose I favour a small party that is not likely to be part of the majority. If I vote for that small party I'm essentially throwing away my vote. Proportional representation does encourage, in general, more voting for small parties than a majoritarian system, but only for small parties that are likely to be part of a coalition.