I'm using flat assembler 1.56 for Win32 GUI (the 14 December build) and found something interesting when compiling the "same" code (2 lines switched place).

Example one

When compiling this example it ALLWAYS takes 1536 bytes (the size of BUFSIZE does not matter)
BUFSIZE = 1024 => 1536 bytes executable
BUFSIZE = 10240 => 1536 bytes executable

Example two - bug?

As you can see the only difference now is that the "buffer" and "letter" line has switched place, and now the executable changes if you change the BUFSIZE!
BUFSIZE = 1024 => 2560 bytes executable
BUFSIZE = 10240 => 11776 bytes executable

I guess this is a bug, unless I've missed some important part
And btw, if "letter db 0" is changed to "letter db ?" then both examples gets the same executable size (1024 bytes).

No, it is not a bug. It is normal behaviour. The uninitialized data (i.e. declared with rb, rw, rd, rq or "db ?", etc. and when it is declared at the end of the data section (after all initialized data) will not be included in the executable file, but will be created directly in memory during loading of PE executable (remember this data memory is only reserved, it doesn't contains defined data).
So, in order to get minimal file size, you MUST place your uninitialized data at the end of the sections.

Regards.

Ah that explains it, thanks.

As you can see you have written code AFTER data, but you still have minimal size exe. In PE file code is right after the header and data is always at the back end, nice optimization concerning size.

often uninitialized data placed in special section