Beyond all other variables, its assured that the average change in the diminished value of a single Healing Surge per additional unit (PC) added is an upward trend that tapers off probably somewhere around 8 (when you have 2 controllers, 2 defenders, 2 leaders, 2 strikers). Cut @
Storminator group down from 7 to 5 and they would feel it. Cut the Paladin out of your group and you would feel it. This is for a number of reasons including monster

C ratio, the atrophying of the ability to dictate target acquisition to foes, the ability for foes to more easily access combat advantage and its potential riders, the inability to potentially cover all roles and/or synergize. Conversely, as all of those grow further in the PCs favor, the inverse is true (such as in the 8 person group).
Not having a dedicated Defender in a group has a lot of implications, up to and including having to synergize harder to dictate enemy target acquisition. For instance, the Bladesinger is effectively the group's Defender and has zero problems performing that role from the damage soak angle. His AC and Reflex rival defenders and he has an array of activatable powers to exceed classic Defender AC and Reflex for about 80 % of combat. Couple that with Stoneskin and his survivability is extreme. His only problem would be his Fort save but its buffed by 2 separate feats to around Defender levels. He has very good single target control (and damage) but when there are a considerable number of non-minion enemies in the combat, things can get hairy. As such, the Rogue is built to be able to raise his AC dramatically on command (Duelist build) and the Druid has a sub-build to help the Bladesinger have better multi-target "stickiness" (Call of the Beast primarily).
Point being, a smaller group has to diversify their individual repertoires rather than focusing them toward one specific end, diluting their primary function; another disadvantage that steepens the amplitude of diminishing Healing Surge value per additional unit.