Используйте OPTICAL_ZOOM как таковой;
VARIANT vaZoomFactor;
VariantInit (&vaZoomFactor);
V_VT(&vaZoomFactor) = VT_I4;
V_I4(&vaZoomFactor) = 50; //-- in this case, 50%
browserCtrl.ExecWB (OLECMDID_OPTICAL_ZOOM, OLECMDEXECOPT_DONTPROMPTUSER, &vaZoomFactor, NULL);