I concur; it seems to have helped expand SEA into a mega-hub.
Seattle airport has lots of good features, but it is not a mega-hub by any stretch. It is only the 8th largest in the USA.
Looking at Mainline #s (because BTS doesn't break down the regionals by operating partner), it is the seventh largest Mainline hub in the country based on pax carried but yet many on this board would consider EWR, SFO, ORD & DEN mega-hubs for their carriers that fall behind on this list.
ATL (DL Only) - 67 million passengers
DFW (AA Only) - 39 million passengers
CLT (AA Only) - 25 million passengers
ORD (UA Only) - 21 million passengers
MDW (WN Only) - 19 million passengers
DEN (UA Only) - 18 million passengers
SEA (AS Only) - 18 million passengers
MSP (DL Only) - 18 million passengers
DEN (WN Only) - 18 million passengers
LAS (WN Only) - 17 million passengers
ORD (AA Only) - 17 million passengers
SFO (UA Only) - 17 million passengers
EWR (UA Only) - 16 million passengers
MIA (AA Only) - 14 million passengers
PHX (AA Only) - 15 million passengers
DTW (DL Only) - 15 million passengers
LAX (AA Only) - 12 million passengers
LAX (DL Only) - 11 million passengers
Sure it doesn't include Regional numbers that are essential to a hub but it gives you an idea of how big the operation for AS actually is.