Show us this proof. Give us a census of the Nords who support the Empire vs. those who do not.
Dawnstar 38 (including 5 Dark Brotherhood members, and 4 non-residents)
Riften: 78 ( including 20 Thieves guild members, and 5 children)
Windhelm: 50 (including one dead NPC and 2 children)
Winterhold: 30 (including 19 Mage guild members)
Stormcloak total: 140 unique NPCs plus 55 guild members and children, and none residents.
http://www.uesp.net/wiki/Skyrim:Falkreath_People: 30 (including 9 Dark brotherhood members)
http://www.uesp.net/wiki/Skyrim:Morthal_People: 18 (including 2 children)
http://www.uesp.net/wiki/Skyrim:Solitude_People: 83 (including 2 children (seems low, i might have missed one) and 3 temporary workers from the East Empire Company)
http://www.uesp.net/wiki/Skyrim:Markarth_People: 85 (including 1 Thalmor agent and 2 children)
Imperial total (without Whiterun) 197 unique NPCs, plus 19 Children, guild members, and non-residents
Whiterun: 79 (including 5 children, 1 Housecarl, and 13 guild members
Imperial total with Whiterun: 257 plus 38 non-combatants etc.
Inevitably the question of divided loyalty will be brought up, clearly the Grey Manes and Battle Borns don't see eye to eye, and thus the entire population of Whiterun would not be given to the Empire. This would be a logically true statement, however such a thing would be too tedious even for me, to sort out and IMO, would harm the Stormcloaks even more as not only do they have an equal division amongst its population centers, but also have the Dark Elf question. I don't think many of them would stay loyal to Windhelm if an opportunity arose. We then also have the issue of towns and villages, again a tedious affair that would likely hurt the Stormcloaks again, as the Imperial Nords control more towns and points of interest (15:10)
No idea how some of them became linked, or how to make the others linked as well. Population can be double checked in the CS, wiki (http://www.uesp.net/wiki/Skyrim:People) or if you're feeling brave, an ingame headcount.
Doing work for Tullius is the worst, i dislike him

much.
He came off as a bit realistic to me, a do whatever it takes type, perhaps even Ulfric's equal in clever strategic scheming. However i felt the same displeasure when dealing with Ulfric and his bearcub general who I can never remember the name of. Particularly at the end of the Stormcloak campaign during the final scene, which to me felt downright dishonorable discounting how Ricket was treated. I didn't quite like Tulliu's attitude at the end either, but he wasn't insulting a dying man with taunts.