Revert changes to default grid widths

@phobson I hadn't noticed this when the gridweight PR landed but the default grid widths ended up changing. I think there's something to be said for your choices (the plots did look nicer) but it was causing problems because changes to the example notebooks that were meant to only add a cell or two produced diffs which touched every figure.

In the interest of a) having the small changes I'm working on now produce informative diffs and b) holding off on doc updates that touch all of the figures, I'm reverting that change in `rcmod.py`.

It's definitely worth revisiting the default style before I release 0.3, but I'd like to have changes that cause widespread changes to the examples happen all at once, just to keep a handle on the size of the repository.