The Serama Council of North America (SCNA) Standard of Perfection calls
for the following acceptable weights based on our pending ABA proposal for acceptance.
Cocks 16 ounces with a 20% variable either way, or
12.8 to 19.2 oz (358-537 gm), without disqualification
Hens - 14 ounces with a 20% variable either way, or
11.3 to 16.8 oz (313-470 gm), without disqualification
Cockerels - 14 ounces with a 20% variable either way, or
11.2 to 16.8 oz (313-470 gm), without disqualification
Pullets - 12 ounces with a 20% variable either way, or
9.6 to 14.4 oz (368 to 403 gm), without disqualification
I read that the average size for a Jap hen is at least 19oz...... so that's larger than a mature serama rooster.
http://nationalzoo.si.edu/Animals/KidsFarm/InTheBarn/Chickens/atthezoo.cfm
says that Jap Males weigh about 1.63 pounds and females weigh about 1.38 pounds.