Log file that `sudo last` accesses?
The file (executable) in question is /var/run/syslog

If we execute man last we get

SEE ALSO
     lastcomm(1), utmpx(5), ac(8)


We can find utmpx in the /var/run directory  Utmpx is a user accounting database of current login information.  The key here is this statement found in the man page for utmpx


  Traditionally, separate files would be used to store the running log
  of the logins and logouts (wtmpx),
       and the last login of each user (lastlogx).  With the availability of the Apple system log facility
       asl(3), these separate files can be replace with log entries, which are automatically generated when
       utmpx entries are written.


So, utmpx is generating log entries to the OS X's syslog function.   Execute the following command:

syslog | grep 'login\['

And you will get a list of all login events since the OS was installed (truncated)

May 26 17:03:12 Allans-iMac login[5572] &lt;Notice&gt;: USER_PROCESS: 5572 ttys003
May 26 17:26:15 Allans-iMac login[5572] &lt;Notice&gt;: DEAD_PROCESS: 5572 ttys003
May 27 10:10:57 Allans-iMac login[5196] &lt;Notice&gt;: DEAD_PROCESS: 5196 ttys002
May 27 10:10:58 Allans-iMac login[4741] &lt;Notice&gt;: DEAD_PROCESS: 4741 ttys001
May 27 10:11:18 Allans-iMac login[6253] &lt;Notice&gt;: USER_PROCESS: 6253 ttys001
May 27 10:25:45 Allans-iMac login[6281] &lt;Notice&gt;: USER_PROCESS: 6281 ttys002
May 27 17:17:15 Allans-iMac login[6281] &lt;Notice&gt;: DEAD_PROCESS: 6281 ttys002
May 27 17:17:21 Allans-iMac login[6253] &lt;Notice&gt;: DEAD_PROCESS: 6253 ttys001
May 28 13:40:06 Allans-iMac login[7123] &lt;Notice&gt;: USER_PROCESS: 7123 ttys001
May 30 09:11:25 Allans-iMac login[1213] &lt;Notice&gt;: USER_PROCESS: 1213 ttys000
May 30 09:11:25 Allans-iMac login[1220] &lt;Notice&gt;: USER_PROCESS: 1220 ttys001
May 30 09:23:19 Allans-iMac login[1220] &lt;Notice&gt;: DEAD_PROCESS: 1220 ttys001
May 30 09:23:19 Allans-iMac login[1213] &lt;Notice&gt;: DEAD_PROCESS: 1213 ttys000
May 30 20:57:16 Allans-iMac login[1695] &lt;Notice&gt;: USER_PROCESS: 1695 ttys000
May 30 20:57:39 Allans-iMac login[1695] &lt;Notice&gt;: DEAD_PROCESS: 1695 ttys000
May 31 22:33:30 Allans-iMac login[2592] &lt;Notice&gt;: USER_PROCESS: 2592 ttys000
Jun  3 16:46:28 Allans-iMac login[2592] &lt;Notice&gt;: DEAD_PROCESS: 2592 ttys000
Jun  4 20:44:43 Allans-iMac login[1789] &lt;Notice&gt;: USER_PROCESS: 1789 ttys000




For additional user accounting info, we can use the command ac

Executing man ac gives us 

    NAME
     ac -- display connect-time accounting

SYNOPSIS
     ac [-d] [-p] [-w file] [users ...]

DESCRIPTION
     A record of individual login and logout times are written to the system log by login(8) and launchd(8),
     respectively.  The program ac examines these records and writes the accumulated connect time (in deci-
     mal hours) for all logins to the standard output.


So, according to man ac login information is written to the system log.  Executing ac gives us "If no arguments are given, ac displays the total amount of login time for all active accounts on the
     system."

$ ac
total     2477.23


To break it out by user:

$ ac -p
testguy      0.04
_mbsetupuser     0.39
allan     2476.74
root         0.07
total     2477.24