BuddyPress.org

Yes I know how this works. I am assuming you are referring to the post input on profiles, for adding activity posts. This is a feature and not a bug.

There is some CSS setting the height of the comment box. When you click into it, some Javascript recognizes that you have clicked into the box and then makes the box taller. You can override all of this via CSS, by just setting a fixed height to the box.

Here is the CSS to add:

#buddypress form#whats-new-form textarea {
height: 50px;
}

Instead of 50px, use the height you want it to be. If that does not work, try adding an !important declaration like this:

#buddypress form#whats-new-form textarea {
height: 50px !important;
}