I've been running large groups for years now. My current group has 8 players plus a DM. I've found that an 8 player group can typically handle an EL of about 2-3 higher than the typical 4 player group can. Why 2-3 rather than just 2 (as the DM would suggest)? Simply put, an 8 player group tends to have very few weak links in the group's abilities and usually has redudant capabilities against any opponent (e.g. several front line goons, several arcane casters, several healers, several trap-busters). With good strategy, such a group is surprisingly robust.
I tend to favour MORE monsters as a general rule (say about 75% of the time) but I will often use a single big nasty foe with a very high CR. The thing with higher than "DMG recommended" CR foes is you have think about how their abilities will affect a large group differently than a smaller group. Watch out for area effects, gaze attacks and other special abilities that can hurt a large group just as easily a smaller one. Also note if your monster is simply too powerful a melee brute for the party to cope with (if yes, consider ways to adjust the circumstances of the encounter to keep it interesting - e.g. your giant might be a bit drunk or stupid and use poor tactics instead of creaming a single foe quickly and moving on to the next one). Be especially careful about special abilities that are too powerful for a lower-level group to counteract.
I've watched an 8 person party shred CRs as high as 7 above their level (admittedly they used solid tactics there). I've also seen "easy" encounters with creatures that have an area or gaze attack decimate them when everyone had lousy dice luck on a save.