Hill-Roberston effects and effective population size
Hopefully this syllogism will answer your question.

Given the following premises:


In the absence of selection, fitness of individuals are at a theoretical maximum.
If a theoretical maximum fitness is achieved then effective population size is maximum.
If there is an allele that confers both increased and decreased fitness you have a genetic conflict (e.g. an allele that turns you green gives you good woodland camouflage but is un-attractive).
If a deleterious allele is linked to a beneficial allele this is similar to having a genetic conflict.


The conclusion follows: 

Strong linkage between beneficial alleles and deleterious alleles (Hill Robertson effects, i.e. 3. and 4.) prevents a theoretically maximum fitness to be attained (1.) and thus a reduced effective population size (2.). Which can only be overcome by high levels of recombination and mating (i.e. by separating deleterious and beneficial alleles).