COM files were not limited to only 64k of accessible memory, but the difference was that upon loading COM file got one full preallocated 64kB segment that got u

COM files were not limited to only 64k of accessible memory, but the difference was that upon loading COM file got one full preallocated 64kB segment that got used for all of data, code and stack, while MZ format files specified in their header how many paragraphs (segment register increments, ie. 16 byte blocks) they need for code and how many additional uninitialized paragraphs should follow that (minimum and maximum required). In both cases programs were free to allocate additional memory from DOS, but by default EXE files were loaded in such way that this wasn't necessary and automatically got the largest continuous block of free memory allocated to them (in fact they got the highest addressed block of free memory, but in any sane state of DOS it was also the largest).