Those cited codes are doing memory alignment, the internal variable "->a" is used to check if there's enough space to hold more data, while a similar check is done by the malloc implementation:
musl: 382-401, 411-414, 424-428
openbsd: 1579-1617, 1678-1688
glibc: 3183-3222, 4573-4623
Those values are not exposed; so if you plan to use any heuristic to avoid brk/mmap calls (for example "->a * 2"), you will need to keep track of the current buffer size (which may be bigger than the necessary).