Perhaps this will help, mATX (mini) and uATX (micro) are 2 different sizes, mATX being larger than uATX and ATX larger than both.
uATX is often refereed to a M (micro) ATX or uATX. If it has a "M" it is most likely micro, if it has a "m" it is mini, I have not seen any mATX boards in a while though, just the MATX (uATX).
Actually, cin19 was correct. mATX and uATX are abbreviations for microATX.
https://en.wikipedia.org/wiki/MicroATX
I am sorry be I disagree, there in in fact a mATX MB because I have used them, they are not uATX. When you see board model numbers listed they say such as"
uATX
ASUS P8Z77-M PRO LGA 1155 Intel Z77 HDMI SATA 6Gb/s USB 3.0 Micro ATX Intel Motherboard
ATX
ASUS P8Z77-V PRO LGA 1155 Intel Z77 HDMI SATA 6Gb/s USB 3.0 ATX Intel Motherboard
There is no mATX associated with a uATX board in model number or specification. If so could you please show me so I don't tell people the wrong information, any source would be fine.
Just an aside: the "u" is short for micro, like the "m" is short for mili and the "n" for nano, I guess that is why micro ATX is often called uATX.