On the Equip soldier screen, clicking on an item, whether in the soldier's inventory (backpack, belt, holster), or in base inventory (stores), allows for displaying informations about this item (bottom left of the screen).

Clicking and dragging an item activates a green display to visualize the free remaining space in the soldier's inventory, as well.

Now, it seems that clicking on an equipped item fools this display, in that the slot where the item you clicked on before lies, ''is included'' in the green shape!

This doesn't occur with hand held items.
All the items are concerned, afaik: grenades, ammo clips, 4-squares items such as MG clip and medipack, 6-squares items like laser and plasma pistols, ...

The bug lasts until a hand held item is clicked on, or until any item is actually moved in the soldier's inventory (to drag it is not sufficient).