The ctrl+f5 key combo has a bit of history behind it.
Stonesense and Dwarf Fortress are both 32bit programs. what this means is that they have a memory usage limit of 2Gb, no matter what your system has.
Originally, Stonesense ran separately from Dwarf Fortress, in it's own process. This meant that it had that 2GB for itself, and if it crashed, it just crashed itself, and DF was fine. Unfortunately, this uses very slow system calls to read the memory from DF, which meant that things tended to run slow in general.
Then a bunch of things happenned. The first was stonesense becoming multi threaded. This meant that the DF memory could be read at the same time that SS was busy drawing pretty pictures. But this meant that two copies of the viewed area of the fort needed to be loaded at once. This meant double the memory usage, but it also was double the speed, so it was good.
However, enough changed with stonesense that the full screenshot no longer worked. I didn't really feel up to the task of getting it to work again, so I disabled it. the main problem was that the loading was no longer sequential, so it was like deciding that you want to cook for 10 times the number of people, while your minion is already at the store buying groceries.
Then DFhack changed, and stuck itself into Dwarf Fortress directly. This means direct access to DF's memory, which means much faster reads, and SS can now read much more than it was able to before. Things like dwarf inventories and mist all came from this.
Unfortunately, the downside of this was that because they are both the same program, the 2GB memory limit is shared between DF and SS, which means when I finally went and fixed the large screenshot feature to work again, the fort pretty much had to be loaded into memory twice, once for DF itself, and once for stonesense. This means that if your fort takes more than a single GB, which most do, then trying to do a full screenshot will cause a crash as everything runs out of available memory. The solution to this was to disable the screenshot feature again because having a crash button is bad.
Finally Caldfir came along, my hero, and fixed the screenshot feature so that it uses far, far less memory than before, making things not crash, making the only limit the size of the resulting image, because videocards have their own limits, but zooming out fixes that, at the cost of detail. This is what the latest version of stonesense uses, but I don't know at which specific version it was implemented, but I can only assume that the version you are using was before this.