I share your concern, and think that Romney will likely get more delegates than Santorum or Gingrich. However, it also seems quite possible that Santorum + Gingrich will get in the neighborhood of 1,300 delegates, sufficient to deprive Romney of a 1,144 delegate majority.
Which means a contested convention.
Which means...I don’t know what happens after that, but Romney is by no means a lock in a convention in which a significant majority of delegates represent voters who pulled the lever against him.
(And that’s one argument against Gingrich-Perry. A Gingrich-Santorum or Santorum-Gingrich ticket might unite their respective factions at the convention, so as to prevail over Romney on a 2nd ballot.)
There's not a chance of them getting more than 1,144 combined if they both stay in the race. Ron Paul already has 64 and will continue to get more. Romney probably gets a bare minimum of 1,050 if everything stays status quo, and could get over 1,144 with just a little improvement. Most likely Romney and Paul will have enough to combine for a win at the convention. The only thing that could change the game is Newt or Rick dropping out and teaming up with the other. Then the remaining one could deny Romney a plurality in anything but Northeastern and Mormon states, and sometimes get over 50%. With 13 states with some form of WTA in April and beyond and only 10 proportional, that is the only way to shut Romney out of enough delegates to make a combined Newt/Rick convention ticket possible.