The top 10, in order:
"Olde" "P" "Interval[{423486510550, 487127920732}]"
"Ilel" "A" "Interval[{300087651600, 353729504664}]"
"Core" "P" "Interval[{285981590300, 349457143361}]"
"Vlan" "K" "Interval[{261401922800, 295658587988}]"
"Vlan" "G" "Interval[{226683343970, 279948777146}]"
"Ustr" "N" "Interval[{234393899100, 275725351374}]"
"Olde" "A" "Interval[{231417099500, 271820110586}]"
"Core" "H" "Interval[{226486093000, 269699544976}]"
"Gash" "A" "Interval[{225747464350, 259859006644}]"
"Ilel" "J" "Interval[{220204604290, 254549286463}]"
(Sector tags being either "Official" or "InReview" or "Preserve" AND "OTU", Milieu=M1105, which is my definition of "official traveller universe")
And, the least populated subsectors that have any population at all:
"Corr" "J" "Interval[{600, 699}]"
"Corr" "I" "Interval[{600, 699}]"
"Rift" "K" "Interval[{520, 727}]"
"Verg" "A" "Interval[{800, 899}]"
"Prov" "L" "Interval[{1100, 1298}]"
"Touc" "I" "Interval[{2000, 2999}]"
"Rift" "E" "Interval[{30000, 39999}]"
"Gate" "E" "Interval[{32000, 42998}]"
"Reft" "G" "Interval[{50000, 59999}]"
"Touc" "G" "Interval[{300000, 399999}]"