Tatami has 4 types:
京間
きょうま
Used in 関西・中国・四国・九州.
1.91mx0.955m, 6 = 10.94㎡
中京間 (or 三六間)
ちゆうきようま
Used in 愛知・岐阜・三重
1.91mx0.955m, 6 = 9.93㎡
Horizontally 3m
Vertically 6m
江戸間 (or 五八間)
えどま
関東
1.76mx0.878m, 6 = 9.27㎡
Vertically 5.8m
団地間 (or 五六間)
だんちま
1.7mx0.85m, 6 = 8.67㎡
Vertically 5.6m
When people in Kansai moved to Kanto, they felt smaller, this is why :-)