While it's too early to tell what the market shares will be - none of the new DL flights have started yet - as far as slots the percentage is quite a bit more lopsided than that. And for the 12 months through last November DL mainline already had a bigger market share than AA mainline by 5 percentage points. I suspect that once all the new slots DL gets are utilized, it will have at least twice the market share of AA, with US having a 5-6% share thanks to the Shuttle.
In other words, a US/AA merger would give the merged carrier about the same share at LGA as DL has before the swapped slots are utilized - still a distant 2nd.
I recently saw a chart (can't find it right now) showing that, after the slot swap and divestitures to jetBlue and Westjet, DL holds about 47% of LGA slots, AA holds about 21% and US holds about 16% (shuttle and flights to eastern hubs). If those numbers are accurate, DL would still hold a big lead on a combined US-AA (at about 37% of LGA slots).
Here's an article mentioning number of slots: http://www.bloomberg.com/news/2011-10-11/delta-wins-approval-to-build-laguardia-hub-as-u-s-backs-flight-slot-trade.html
I think the chart was linked by the teenager on Flyertalk (adam?) who said that DL should segregate all domestic flights at LGA and all international flights at JFK and force connecting passengers to travel between the two. The moment the press latched on the possibility that DL might bid on AA, he posted dozens of times in the DL forum that it was a done deal and that DL would own AA very soon.
IMO, the slot swap was the biggest blunder of Parker's career. He gave away 132 slot pairs worth, potentially, hundreds of millions of dollars based on the prices in the divestiture process. That was poor pricing, but the huge blunder part was that a combined US-AA won't be able to keep many (if any) of AA's DCA slots, as US now holds probably the max. Had Parker not been so hell-bent to make the big gift to Delta, and kept the LGA slots, and US and AA combined, he would have received plenty of DCA slots held by AA plus the combined AA-US would hold the maximum number of LGA slots permitted by the antitrust enforcers (would have had to divest some).
Today, DL reported a gain of $43 million on the slot divestitures, which paid 2/3 of the cash DL gave US.
By giving away so many LGA slots, Parker guaranteed that a combined US-AA would be hobbled at LGA compared to DL. Most. Stupid. Move. Ever.
Parker may not have been able to make the most of his LGA slots, but that's never been a problem for AA. If Parker takes over AA, to whom will he give away AA's LGA slots?