Fabio Riccardo (an important early advertiser of ours, and part of the reason TOP survived the aughts) wrote a nice comparison of the X-T1 and the Sony A7 that I added as a "Featured Comment" to the first "X-T1 Size" post, below.
It seems to me that the X-T1, X-Pro1, OM-D E-M5, and Sony A7 models are all close enough to "the same size" as really makes no difference unless you're being finicky and fastidious (which, of course, we all often are when we appraise cameras for fun, but never mind that for now).
The real difference, as Fabio points out, is the lenses. The larger the sensor, the larger the lenses, all else being equal. It's true that it's possible to design smaller lenses for larger sensors, but the designer is under heavier constraints, size and selling price being the top two (of many) constraints on the freedom of lens designers.
In this three-way comparison, Sony's tendency not to build up lens lines has to be brought into the equation. In fact, with the announcement yesterday of the A7s, a 12-MP variant with 4K video (read more about it here) and the third camera in the A7 line, Sony is actually close to making more cameras for the A7 lineup than lenses(!).
Of course you can still use adapted lenses. Part of the fun of mirrorless.
Micro 4/3 still really makes the most sense if you want the most flexible compact mirrorless system. But of course half the fun is the choice, and, as Fabio discusses in his comment, now we have three sensor sizes to choose from (four if you want to include 1"): Olympus and Panasonic in 4/3 sensor size, Fuji and Sony NEX (or ex-NEX, as I now call it) et al. in APS-C, and Sony in FF. If you're looking for a compact mirrorless system to commit to, you've got a lot of considerations to weigh, but it's a fun and interesting choice to be able to make.
(Thanks to Fabio)
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(To see all the comments, click on the "Comments" link below.)
Featured Comments from:
Glenn Brown: "Size matters. I have used the Fujifilm X cameras professionally since the X-Pro-1 came out, and love the system. Last week I had a tough very wide angle assignment and borrowed a friend's D4 with 14mm, and laughed on how big it is, and shake my head at the backbreaking heft of it. Ordered the Fuji new ultrawide zoom today."
Sixblockseast (partial comment): "Part of me thinks Sony comes out with all those A7s just to showcase their sensors which they then sell to other camera makers. Hence no need for lenses."