As empati almost correctly points out - the a/c rotates (flares) - not only about the lateral axis, but about the CG. One should think. However; during a flare, especially a desperate save-the-day-flare, the instantaneous point of rotation is not about the CG. A long B737 will usually rotate about a point ahead of the CG. If you attach a spring to a model aircraft; right above the CG, and push quickly down on the tail, you´ll see the spring extend. This extension will be greater on a 738 than on a 733. This ´cause og th elonger distance (arm) btwn CG and tail. The undercarriage usually sits behind the CG, and a rapid flare will further increase the arm between the undercarriace and the point of rotation. You just make a bad situation worse.
So, as RAT 5 has expertly observed; even though the distance btwn CG and undercarriage on the 733/738 is pretty much the same, the longer CG-tail arm on the 738 will cause a geater down-force on the undercarriage. A forward push on the column is thus more likely to save the day than is a "traditional" flare.
I used to fly the Dash-8 300 and Q400. Same-same, but different. An extra 5-7 kts on short final always did the trick on the Q400
