Males of the hermit crab Pagurus filholi show pre-copulatory mate guarding behaviour to keep a receptive female from other rival males during the breeding season. Guarding males are often replaced by other males via contest competition. Shell size fit and shell species of males effects were investigated to see if their ability to defend females against rivals depends on their shell. Shell size fit and shell species of guarding males were experimentally manipulated and then, the males were allowed to guard a female and interact with rival males of various sizes. Contest outcomes depended on neither the shell size fit nor shell species of the guarding males, and depended on body size ratio between guarding and rival males. When the body size of guarding and rival males was similar, however, guarding males in large fitting shells defended their female mate significantly more often than those in small fitting shells. For Chlorostoma lischkei shells, small males preferred larger shells during the breeding season than after the breeding season. The plasticity in the shell size preference of small males suggests that they compensate for body size disadvantage in mating versus larger rivals and for higher defence ability of female mates versus similar sized rivals.