Part of my point was that going after Exxon and Walmart, would be extremely difficult because of their very strong financial health (not to mention connections and influence).
Gnats don't concern Exxon, they get swatted. At that scale, Icahn is a gnat. Exxon would have to be extremely mismanaged to inspire so many investors to line-up for a serious proxy battle. Simply doesn't happen unless a company is on its back, or knee deep in a huge scandal relating to the leadership.
If you buy a billion dollars worth of Exxon and try to stir up trouble, you'll be completely ignored unless the company is being very poorly managed such that much larger fish than you are willing to join your cause.
With eBay, since Icahn is a relatively small fish (in terms of what he can personally leverage into the battle) trying to force the hand of a $75 billion enterprise, he's going to need much larger fish than himself to join his cause. He simply can't do it himself. That's the other point I was trying to make about their scale. The larger the company you take on, the larger the total resources you're going to have to get lined up on the same agenda.