I've never understood why people think this way.
We don't need limits on ships. As Sanchez said, just throw the same supply worth of smaller ships at the Titan, or if Titans are actually cost-effective, just throw the same number of Titans.
I think the sticky point is that we have to come up with a good way to judge cost effectiveness. Is it based on the cost of supply? Research? Unit cost?
For example, I'd say that equal supply of, say, heavy cruisers, would lose to a Titan. But equal resource cost of said heavy cruisers would win.
TBH, I think that's somewhat similar to how a capital ship handles; equal supply of heavy cruisers is around 2-3, something a capital ship could at least run away from, if not stalemate or perhaps defeat. Equal cost, OTOH, nets you about 5-6 heavy cruisers, making it far more likely for said HCs to beat the snot out of a capital ship.
EDIT: also wanted to chip in my opinion that Titans would be self-limiting by virtue of logistics and resource/research costs. Fielding one Titan would probably be somewhat similar to fielding two or three capital ships, or perhaps a superweapon. Fielding two? Doable, but it'll put a strain on your fleet supply and probably your resource logistics. Three or more? Forget about it, you're better off with cheaper and more numerous ships that can be more places at once.