I think the explanation is a bit different. This is just my viewpoint, it may be far from the truth though :
Lone pairs indeed need to be considered. But remember, that Cl- has 3 lone pairs even after coordination. And water has only one after coordination. The effect of decrease in Δ when π-donor ligands are considered, I hope, is known to you. Hence when you remove one LP (in going from Cl- to water), the decrease in Δ if lifted and hence there is an increase.
The amount of increase is lesser with the Cr3+ complex because Cr is in a higher oxidation state, in which the dπ system has only 2 electrons. As against the Fe2+ complex which has 6 electrons in its d system. Hence the greater relief of repulsion, which means, greater increase.