[pygtk] Prevent expansion of combobox in a table

[Durand]

Hi,

I've got this really annoying problem where my combobox is just filling up  
the space it gets rather than sticking to it's size like the textbox next  
to it.
Screenshots:  
http://img36.imageshack.us/img36/6890/screenshotkanjilookup1.png  
http://img4.imageshack.us/img4/9200/screenshotkanjilookupu.png
The code is pastebinned here: http://pastebin.com/3DXCetvn
though it's pretty simple really. I've got a table with 4 columns and 2  
rows and a textbox fills 3 columns in one row and the combobox fills the  
other column in the one row. The second row contains other stuff. I'm not  
really sure why this isn't working. If the second row contents expand  
fully, then the combobox sorts itself out, otherwise it doesn't

Can someone help please? Thanks!