> can't strace trace brk() calls?

Absolutely.

> what is the easiest way to trace all allocations (brk() and mmap) but nothing else?

  strace -e mmap "$command"

I don't think anything modern still uses the program break but one should know brk and sbrk exist. To see deallocations, add munmap to the filter. Note that these represent operating system allocations: programs usually request huge chunks and then manage that memory in user space in order to avoid system call overhead. In many systems, this memory won't actually count as used unless the process actually touches it and causes page fault.