Also known as: 粗壮的脖子
Necks in anime are usually proportional to the overall body of the character, or sometimes unusually smaller. This is especially true for female characters, probably because thick necks are considered a more masculine trait that older and more Muscular men have.
Sometimes thick necks are also used for comedic purposes, like with Tanigaki Genjirou from Golden Kamuy.
Or simply as a character design choice. This is especially common in One Piece.