A lot of /b/ is comprised of noise, it's mainly a few very prolific users who use it as a rapid-fire chat room - whereas the rest of 4chan is used more like a message board. So of course, b has more posts. But even so, it's not true that most of the action on 4chan is /b/, and certainly not most of the users.
/a/ (anime) 76,358,226
/b/ (random) 443,355,941
/c/ (anime/cute) 1,952,748
/d/ (hentai) 4,429,215
/e/ (ecchi) 1,531,210
/f/ (flash) 1,830,359
/g/ (technology) 29,833,456
/t/ (torrents) 541,357
/u/ (yuri) 1,321,534
/v/ (video games) 166,666,985
/vg/ (video games) 20,810,830
/w/ (anime wallp.) 1,444,187
/wg/ (wallpapers) 4,667,232
/i/ (oekaki) 398,648
/ic/ (artwork crit.) 1,083,696
/r9k/ (robot 9001) 4,798,295
/cm/ (cute/male) 2,802,217
/hm/ (handsome men) 446,647
/y/ (yaoi) 1,990,139
/3/ (3dcg) 319,975
/adv/ (advice) 11,246,991
/an/ (animals&nat.) 1,327,885
/cgl/ (cosplay&egl) 6,486,836
/ck/ (food&cooking) 4,071,583
/co/ (comics&carts.) 43,931,256
/diy/ (do-it-yourself) 351,467
/fa/ (fashion) 5,271,417
/fit/ (health&fitness) 16,363,177
/hc/ (hardcore) 301,394
/int/ (international) 7,199,819
/jp/ (otaku) 10,207,786
/s/ (sexy) 13,779,843
/lit/ (literature) 3,241,126
/mlp/ (pony) 6,535,229
/mu/ (music) 31,060,238
/n/ (transport) 423,302
/po/ (papercraft) 485,531
/pol/ (pol. incorr.) 8,219,557
/sci/ (science) 5,348,462
/soc/ (social) 14,029,667
/sp/ (sports) 29,263,446
/tg/ (trad. games) 22,025,427
/toy/ (toys) 3,233,423
/trv/ (travel) 661,343
/tv/ (television) 28,024,181
/vp/ (pokemon) 10,674,534
/wsg/ (worksafe gif) 84,034
/x/ (paranormal) 11,544,411
/q/ (meta disc.) 328,594
TOTAL 1,062,304,856