Wow, I touched a nerve I don't understand, to get a -1, so I'll explain myself. The GNU code looks like it's portable to a bunch of OSes, some with and some without a given feature. That sort of portability leads to having a lot of #ifdefs. If you're only going to support a handful of OSes - because that's all most people care about - then of course your code is going to be smaller.

As a concrete example, the linked-to "who" uses:

    if (entry.ut_type != USER_PROCESS)
      continue;

The GNU one uses:

      if (IS_USER_PROCESS (utmp_buf))

The IS_USER_PROCESS() macro is in gnulib's readutmp.h (which would need to be included in the overall line cound):

   # define IS_USER_PROCESS(U)                                     \
      (UT_USER (U)[0]                                              \
       && (UT_TYPE_USER_PROCESS (U)                                \
           || (UT_TYPE_NOT_DEFINED && UT_TIME_MEMBER (U) != 0)))

The UT_USER maps to ut_user on some systems, and ut_name on others:

   /* Accessor macro for the member named ut_user or ut_name.  */
   # if HAVE_UTMPX_H
   
   #  if HAVE_STRUCT_UTMPX_UT_USER
   #   define UT_USER(Utmp) ((Utmp)->ut_user)
   #  endif
   #  if HAVE_STRUCT_UTMPX_UT_NAME
   #   undef UT_USER
   #   define UT_USER(Utmp) ((Utmp)->ut_name)
   #  endif

This is because, as https://www.gnu.org/software/libc/manual/html_node/Logging-I... points out, "Note that the ut_user member of struct utmp is called ut_name in BSD. Therefore, ut_name is defined as an alias for ut_user in utmp.h"

Each of those other macros supports cases where the machine supports a given test, and when it doesn't. Including for machines which don't implement ut_type! (That's what the UT_TYPE_NOT_DEFINED case is for, which falls back to using the time field ... which has its own set of variants!)

Portability is neither options nor cleverness, so I added it to the list of considerations. But then again, most people now don't need to support decades of history and scores of Unix variants.

If you don't care about all the features then Busybox's implementation will work fine at 170 lines inclusive of comments.