737's spoilers are pretty ineffective at slow speeds
All spoilers are ineffective at slow speeds 'cos there's fewer bernoullis flowing over them, compared to high speed. Also, whilst spoilers do help to slow the aircraft down they are more about putting weight on the wheels as opposed to creating form drag to reduce speed. The brakes do most of the work (assisted by the spoilers) and reverse thrust helps to keep the brake temps down.
I would suggest the reason for you seeing 767s and 747s slow down more effectively than the 737 is because most, certainly not all, 767s, 747s etc have carbon brakes while the majority (but not all) 737s have steel brakes. These 'fade' quicker and have poorer performance when it comes to stopping an aircraft than their more modern brothers. Also the older 737s had a pretty crap, high aspect ratio wing, compared to the newer Boeings, so the Vref is really quite high and I've certainly had higher Vrefs in the 737 than I did in the 744 or 767 (obviously quite light). Higher touchdown speeds, coupled with steel brakes, means no surprises when the newer boys exit the runway sooner.