Why does the WinAPI use an int (32 bits) for the BOOL type?
Wow, slow down a little bit there. First of all, I'm pretty sure programmers have been using 4-byte ints for boolean variables since the beginning of programming on x86. (There used to be no such thing as a bool datatype). And I'd venture to guess that this same typedef is in the Windows 3.1 <Windows.h>.
Second, you need to understand a bit more about the architecture. You have a 32-bit machine, which means that all of the CPU registers are 4-bytes or 32-bits wide. So for most memory accesses, it is more efficient to store and access 4-byte values than it is for a 1-byte value.
If you have four 1-byte boolean variables packed into one 4-byte chunk of memory, three of those are not DWORD (4-byte) aligned. This means the CPU / memory controller actually has to do more work to get the value.
And before you go smashing on MS for making that "wasteful" typedef. Consider this: Under the hood, most compilers (probabily) still implement the bool datatype as a 4-byte int for the same reasons I just mentioned. Try it in gcc, and take a look at the map file. I bet I am right.
Firstly, the type used in the system API has to be as language-independent as possible, because that API will be used by a multitude of programming languages. For this reason, any "conceptual" types that might either not exist in some languages or might be implemented differently in other languages are out of question. For example, bool fits into that category. On top of that, in a system API it is a very good idea to keep the number of interface types to a minimum. Anything that can be represented by int should be represented by int.
Secondly, your assertion about this being "a waste of memory" makes no sense whatsoever. In order to become "a waste of memory" one would have to build an aggregate data type that involves an extremely large number of BOOL elements. Windows API uses no such data types. If you built such wasteful data type in your program, it is actually your fault. Meanwhile, Windows API does not in any way force you to store your boolean values in BOOL type. You can use bytes and even bits for that purpose. In other words, BOOL is a purely interface type. Object of BOOLtype normally don't occupy any long-term memory at all, if you are using it correctly.
The processor is 32 bit, and has a special flag when it operates on a zero integer, making testing for 32 bit boolean values very, very, very fast.
Testing for a 1 bit, or one byte boolean value is going to be many times slower.
If you are worried about memory space, then you might worry about 4 byte bool variables.
Most programmers, however, are more worried about performance, and thus the default is to use the faster 32 bit bool.
You might be able to get your compiler to optimize for memory usage if this bothers you.