I think that it's the grammar rules that make English fairly efficient, and also effectively add to its vocabulary, because word order in English is so strong that it twists whatever word you put in a particular position into whatever part of speech it needs to be. In less order-based languages you have to learn a bunch of elaborate formulas to change the word; in English, it's basically just Subject, Verb, Object and a few mathematical rules about commas, prepositions and conjunctions to make that basic structure recursive or flowery. All of English's core vocabulary is horrifyingly irregular, and you can ignore almost all of that irregularity, prefixes and suffixes, and still make yourself pretty easily understood.

I think the massive vocabulary of English makes it less dense. We have a lot of words that mean exactly the same thing, but that we pretend mean different things. We find ourselves arguing about the innuendo around words or the "actual definitions" of words almost more than material issues, constantly invoking contradictory authorities. We're also constantly repeating ourselves and pretending like we've understood because using obscure words is a sign of intelligence, and not understanding them a sign of stupidity.

Romance languages seem to have a lot fewer words with specific meanings, and do most of their playing with grammar (making a lot of sentences into horrifying mazes.) And irt the center and north in Germanic Europe, the most interesting thing about "Uncleftish Beholding" is that to English speakers it reads like stupid cavemen talking about magic, but it's basically a unit-by-unit direct translation of how Danes, Norwegians and Swedes really talk. English speakers never use the common word for anything if they want to sound like they know what they're talking about.

an aside: I've always heard that Vietnamese is the most efficient by syllable. IIRC some paper came up with a metric where it was about 1.4x more efficient than English. There was also someone who figured out that universally, human languages expressed ideas at the same rate, but the people who spoke less efficient languages just talked faster.

They could have meant different things at some point in time. A Language can change drastically with time and if the people using the words stop differentiating between them it may appear as if it was only a game of pretend. This only becomes a problem when someone that doesn't see a difference in meaning communicates with someone that does.