If the collection's size is smaller than the remaining (allocated) amount of works, MP will definitely hit EOF, never switch to CBR in that case.

Example:

We currently have 10k works for the initial trial phase, split among the number of plans. Let's say 2 plans. Each plan is then given 5k works to find a result or CBR will be tried. If collection size is 5001 docs, trying CBR is a waste of time because MP was one work away from hitting EOF.