In AIX /usr is designed to be a "read mostly" file system. Log files and other things that grow are kept in /var. /usr should only contain executable code. So, in most systems it is perfectly all right for /usr to be 100% full. And you still have a little room left anyway, it just rounded up to 100%. Other file systems I worry at a lower level 80% or 50%, but /usr is ok being full. If you need to install IBM software or fixes, it will expand /usr for you.
But, if you want to expand /usr (or any file system) for some reason, you can do it in smit and change the size to what you want it to be. Often for me I just want it to be a little bigger, so I just increase it by 1 block. Since it can't do just one block, it increases it by the LP size, typically 4, 8, or 16 MB (depends on the size of your disk).
To do that you can run the command :
chfs -a size=+1 /usr
this will increase the size of the filesystem by one LP. By specifying the +1, you are increasing it by one block and you don't have to know what the size is now.
And in support of the previous post, DO NOT increase the logical volume. Increase the file system which will increase the logical volume for you. I have seen people who have filled up their entire disks with logical volumes but didn't expand their filesystems so they were still out of space.
Good luck.