While I appreciate how much value you can get out of repurposing the SharePoint control templates, I do repeatedly get annoyed at using them. Not only they produce an infinite number of tables on your page, they also often hardwire properties for no obvious reason.
This time I came across this problem: we had to put a SPGridView into an InputFormControls and it just did not want to use the full width of the section, as you can see here:
I'll cut it short... it was the ms-inputformcontrols class that hardwired it to 375px. Overwrite it to 100% and you should be fine.