In real-mode DOS the situation is that the program is loaded to continuous block of memory that is followed by it's BSS and heap. The MZ header only specifies how big the BSS+heap part should be. Typical MZ format EXE has the size specified as BSS size by minimum and no maximum, which causes the MZ loader to allocate everything from PSP to the end of present conventional memory to the process. One consequence of that is that if you want to run other programs you have to either change the usual default of "unlimited" heap size or free some memory manually (or use COM format executable which by default only gets one DS=CS=SS segment).