Here's a doubly embarrassing confession. First it involves my use of a construction that I love to make fun of. Secondly my spontaneously generated example is unfortunately also a true sentence.
I was trying on four dresses that have been stored in the attic for a while to see if I could avoid having to shop for a formal dress in Chicago on Friday for the Friday black tie dinner that precedes the Saturday honorary doctorate. I didn't think I was going to be able to fit into any of them, since I've gained back all the weight I lost around 2008-9 and am now close to an all-time maximum. But to my in some ways happy surprise, I found that I could sort of fit into two of them, including the best one. And my surprise was expressed (just talking silently to myself, but obviously in real sentences, since this sentence immediately caught my attention as soon as I "said" it) as "Gosh, I've been fatter for longer than I thought". (The happy part is I may not have to go shopping on Friday, or at least it won't be obligatory to buy a new dress, which takes off the pressure that accompanies last-minute obligatory shopping.)
I still reject that sentence, even though I said it .
I consider most sentences with two 'more's and one 'than' to be either semantically anomalous or else ambiguous among a bunch of readings that could be generated involving various sorts of ellipsis. I think clever advertisers love them and often use them, because they SOUND stronger than a sentence with just one 'more', but to try to pin down a claim of false advertising would be really hard because what they mean is much less clear. (From around 1958: More people get more satisfaction out of L&M than (out of) any other cigarette. From H&R Block signs on their stores in recent decades: "We get bigger refunds for more people."
I know (I think) what I was thinking when I 'said' that sentence, even though I can't see any way to get this as the actual meaning by any compositional semantic rules I know of: [Over all this time that I've been overweight,] I guess I've been up toward my highest weight for more of that time than I realized.
It's not the same as "I've been fat for longer than I thought" — I know that I've been overweight pretty much ever since I quit smoking in about 1986, the only real exception being for a period around 2008-10 when I really worked on it. The "fatter" rather than "fat" has to do with being up in the upper range of my overweight range.
And this definitely isn't an example that could be analyzed via conjunction as some double-'more' sentences seem to be. [One reading of "John put more marbles in more boxes than anyone else" is said to be "J put more marbles in boxes than anyone else and he put marbles in more boxes than anyone else"] Because it's false that "I've been fatter than I thought" — I know how fat I've been — and it's false that "I've been fat for longer than I thought" for reasons given above. So it's certainly not the conjunction of those two, since I wasn't asserting either one.
Oh, I hate this post! But it's all true and it's probably good for me to say it. I hope it will motivate me to get myself back into better shape again. And I think the endless interestingness of this problematic but frequent construction is worth a little embarrassment.