What immediate structural change resulted from using the chain drive on the safety bicycle?

The rider’s center of gravity was dramatically lowered

The introduction of the chain drive mechanism had profound consequences for the bicycle's geometry, moving away from the high-wheel configuration. Because the gearing system managed the speed ratio, the driving wheel no longer dictated the frame size or rider height. This engineering reality allowed inventors to use wheels of equal, small size, similar to modern bicycles. Crucially, this equalization allowed the saddle and the rider's seating position to be placed much closer to the ground. This significantly lowered the center of gravity, resulting in an inherently stable machine far less prone to the catastrophic forward pitches that characterized earlier designs.