Well 5 guys does not mean that things die 5 times faster. Yes they do kill things faster than a solo guy, but you have to look at how fast you can put out the damage and the biggest factor is how many times you have to cast. For example. Lets say a mob takes 6 spells to kill. And for ease of math and all things equal we will use a 2 second casting spell so we don't have to factor in the global cool down. That will take the solo guy 12s to kill the mob. A Duo, 6seconds. a group of 3, 4 seconds. Now here is where things get goofy. Group of 4 still takes 4 seconds as does a group of 5. So a 3, 4, or 5 man group kills the mob in the same ammount of time. So 4 seconds over 12 seconds is only 3 times as fast as solo in this example. Thus it will take longer for a 5 man group to get all the drops (3 times faster, but 5 times the drops needed), they take a penalty to XP by grouping.

Solo: 12seconds per kill for 10 drops = 120seconds
Duo: 6 seconds per kill for 20 drops = 120 seconds
3man: 4seconds per kill for 30 drops = 120 seconds
4man: 4seconds per kill for 40 drops = 160seconds
Full group: 4seconds per kill for 50 drops = 200 seconds

Now lets factor in that each member adds an additional multiplier for running around. A duo will have 2x the run around time getting to the mobs. Thus a full group will have 5x the time of running around. Lets say 10seconds to get from mob to mob. now it looks like this.

220s
320s
420s
560s
700s

This is all things being equal. It doesn't seem unreasonable as right now it takes my 5 man group 2 castings to kill normal mobs. But all this maths is nothing more than to show that a full group doesn't do things 5 times faster.