重 here is read chóng ('again / repeated'), not zhòng ('heavy'). The logic: 重孙 = 'a grandson at one more remove' = great-grandson. Same chóng appears in 重复 ('repeat') and 重新 ('anew').
culture
Living to see your 重孙 is a major Chinese marker of longevity and family fortune - four generations under one roof (四世同堂) is the classic auspicious image, immortalized in Lao She's novel of the same name.
