You may just look at the picture and say "well, duh!" and not need an explanation, but here it is for those that want it...
Both of those controllers came without the headers attached. The 32u, on the right, I did the standard way with male pins underneath and it occupies 30 pins - 15 on either side of center and the overhang takes a couple of additional ones out of play for a lot of uses, so the board is right at half filled or slightly more. The Pro Mini is only occupying 6 pins on one side, leaving a lot more room because I used female pins on top, adding more wiring connections to board instead of taking them away.
There are advantage to each. The 32u is a lot more secure and it is easier to see what the pin labels are. I have a gender bender (just two strips of 6 male pins soldered together) for my FTDI cable, so I can move the ProMini forward one hole, slip the gender bender in behind it and program it while on the board, but it is a whole lot easier to cable to the 32u. But I have some components (like a 434 MHz receiver) that won't fit on the board with the 32u.
Anyway, something to think about before just automatically soldering male pins on minis and breakout boards.