I think, the role of spleen is to kill the infected cells of the body but in case of germ- free mice the infection is minimum that might be possible the function of spleen is less as compared to SPF mice, so the size of spleen in germ free mice is less than SPF mice.

Dear Amrit Kumar

At first, I thought it might be caused by something derived from gut microbiota, but as you said, the absence of infection is certainly the biggest factor.

Thanks.