I have to ask, MIA, if the 737-800's you watch, always start their takeoff run from the same point at the larger jets? (If you can't see the other end of the runway, they not be starting off from the same point. Also, which airlines frequent your local airport? Some may have a policy of "derating" their engines, to reduce engine wear and tear (Engine takeoff thrust can be reduced by programming the Flight Management System). If the runway is long and dry, there should be no problems extending the takeoff run.
Hope this helps.
Regards.
Q.