1st thing is about ionic strength. It's the measure of total effect of all the ions in solution. It's the sum of concentration*square of valency of all ions in solution.
2nd thing is about Ka.
Ka = a[R-].a[H+]/a[RH] where a[X] is effective concentration, i.e activity of X.
a[X] = activity coefficient of X*[X]
But activity coefficient depends on the radius of the ion, valency, and the total ionic strength of solution.
As the ionic strength increases, the activity coefficient decreases (I think there is a formula for the relation between them). So Ka will change due to this change.