S&W makes a full size(FS, 4,5" barrel) 45 M&P. They make a 45c (Not stamped c on the slide, but the barrel is 4"). The 45 M&P frames vary in grip length. The slide rail lengths on both 45 frames are identical. This permits assembling a 4.5" FS barrel/slide on a compact frame. Some are calling this mashup a midsize. Could this be what your rumors refer to?
BTW, S&W sells this "Midsize 45 M&P" from the factory. A lot of posters like it.
Also BTW, the M&P 9/357sig/40 full size and compact frames have different slide rail lengths, so this mid size conversion is not possible, at least not now.
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