Some commentators suggest that the heaviness of Eli is mentioned to create a contrast between him and the glory of the Lord. The Hebrew word translated as heavy
is the same word that can also be translated as glory.
Eli fattened himself on the choice of the offerings made to the Lord (1 Samuel 2:29). He was thus unconcerned for the Lord’s glory. Now the glory of the Lord has departed and the heaviness (glory) of Eli results in his death.1
18 As soon as he mentioned the ark of God, Eli fell over backward from his seat by the side of the gate, and his neck was broken and he died, for the man was old and heavy. He had judged Israel forty years.