I'm not sure if that can be circumvented. I don't really use subforms so I hardly ever play around with them. I think it's a default setting, it has to fill up the rest of space to fit your screen. Post your db let me have a look.
I've just had a quick look and tried to adjust it. That handles the overspill from your controls. It's either you increase the width of your subform or write some code to adjust the widths of all controls before loading. However, even with that once your increase the width of any field, it will show if it overspills. Have a quick search on google for setting the widths of datasheet columns using vba.
We're going off tangent on this thread and like I mentioned in my last post, if you had any further questions, create a new thread so this thread stays focussed on the OP.