It is the ratio of speed to size that matters.
I created what I call a Forty Freak Score. It attempts to scale up to compare bigger players with a 200 lbs player that runs a 4.4 forty.
Anything over Zero is similar to a player that weights 200 lbs running faster than a 4.4 forty. A negative number is similar to a player that weighs 200 lbs running slower than a 4.4 forty.
It is the last column on the right.
IMO, measureables are useful for differentiating players with similar demonstrated performance on the field.
It's like having an extra scouting report.
(click twice to enlarge)