Player weight

From what I'm reading in this forum I get the impresson there are no weight guidelines for the players. Player weight, I assume, should not be extreme as in comparison to the other players. Like in professional football a RB will not weigh the same as a O-Lineman. The weights would vary by position although you occasionally get the anomalies like William "Refrigerator" Perry,
or a big fast running back Jerome Bettis, the late "Ironhead" Heywood, or the small fast guys ie Reggie Bush. Am I correct on this point?