Idle in transcation problem?

i use the "ps -ef | grep postgres" to see all the connections. the
connection's  status is "Idle in transcation".

and i use the " du -h /mydb" to check the disk size. The disk size of
mydb has increased from 400MB to 600MB.
Why? because of the "idle in transcation" ?



my pg version:8.1.3
OS: Linux Enterprise 4