Actually since the neutrals run to the same neutral bar it is only after you get to the point of where the circuits split up that you have to pay attention to which neutral is paired with which hots, which would remove any reason running a single over sized neutral, since after this point you would need separate neutrals anyways, If I have gray on hand I will use gray for one MWBC and white for the other, but with the new requirements of 2011 and even the changes to the 2008 kind of makes this a mute point, and GFCI breakers and AFCI's would also not like it unless you were using the new GE AFCI's and I don't know of or haven't seen a 3-pole GFCI breaker?
The only time I used this was when I relocated a service panel, and made the old box a junction box leaving the old neutral bar in place, and I just ran all the hots to the new panel, and one neutral sized to the old panel neutral bar to pick up the existing neutrals, saved some wire, but that was some years ago, and I remember discussing it here, and remember some didn't like it, but back then there wasn't any code that prevented it. many all in one manual transfer panels with the load center also did this from the factory. the other place I saw this as factory wiring was office cubicle wall assembles where they would have 4 or 5 circuits with one or two over sized neutrals in the feed flex. but the 2011 ended it.
Wayne A. From: N.W.Indiana
Be Fair, Be Safe
Just don't be fairly safe