J.R. S. answered 04/15/19
Ph.D. University Professor with 10+ years Tutoring Experience
(1) Mg(OH)2 + 2HCl ===> MgCl2 + 2H2O
(2) HCl + NaOH ===> NaCl + H2O
These are the two titrations being used.
In (1) there is excess HCl. Some HCl neutralizes the Mg(OH)2 and the xs HCl is titrated in (2) with NaOH.
If there is more Mg(OH)2 to begin with, but the same volume of HCl is used, that leaves LESS HCl left over to titrate with the NaOH in #(2) above. Thus, the volume of NaOH needed to back titrate will be DECREASED.