It literally seems to translate, "He is in long (length) more big (or large)." How about, "He is more big."? Although I can see now that, "much bigger" sounds more natural i Bearla, even if it puts part of the comparative into the word, "big" and leaves the degree of the comparative in the word, "much". Is the, "i bhfad" part idiom, because I don't see why it wouldn't be, "Tá sé níos mó."
Can I turn the question around, and ask why do you think it should be "there is much more"?
There is a quirk in English in that, with an indefinite article, it sounds better to say "there is an apple in the bag", but if you change the article from indefinite to definite, you say "the apple is in the bag".
It is possible to say "an apple is in the bag" or "there is the apple in the bag", but both of those sentences are a bit awkward, or only work in very specific contexts.
Irish doesn't have that quirk, so you have
tá úll sa mhála - "there is an apple in the bag"
tá an t-úll sa mhála - "the apple is in the bag"
tá madra sa chlós - "there is a dog in the yard"
tá an madra sa chlós - "the dog is in the yard"
Why does English use "there is" in these sentences?
In this case we have Tá sé i bhfad níos mó. Well, there clearly isn't a definite article before sé, but that's because you can't put a definite article before a pronoun, and tá sé is always going to be "it/he is" rather than "there is" (except when sé means "six", and tá sé mhadra sa chlós means "there are 6 dogs in the yard"!)
So basically, the reason this cannot translate as "there is" is because tá only becomes "there is" before an indefinite article, but sé is a pronoun, and you can't put an article before a pronoun.
Having said all that, you can't put a indefinite article before "much" either, but you do have an indefinite article in the equivalent "a lot" - "there is a lot more", and the Irish for "there is much more" or "there is a lot more" is Tá i bhfad níos mó ann.
Why ann? You need the preposition to anchor the statement - it just doesn't work without it. Some people will translate that ann, and get "there is much more in it" or "there is much more to it" or "there is much more there", but "there is much more" is an equally valid translation.
And while we're talking about definite and indefinite articles, the Irish for "there is too much" or "there are too many" is tá an iomarca ann.
tá an iomarca bainne sa chaife - "there's too much milk in the coffee"
tá an iomarca daoine ann! - "there are too many people!"
tá an iomarca milseán ite agam - "I have eaten too many sweets"
You asked, “ Why does English use "there is" in these sentences?”
To take your question a step further, there’s a similar “hitch” in English when talking about weather or temperature. We say “It is raining” to mean that rain is falling. What is the “it” in that sentence? We can’t say that the weather is raining, because weather can’t do anything (okay, it can change, but that’s about it).
We also say “It is hot in here”. We’re describing temperature, but temperature can’t be hot or cold. It can be measured in degrees, either specifically or generally.
It’s one of the idiosyncrasies of English....
Did you expect not to be confused by vocabulary in a language that you are learning?
It's much/a lot/a fair bit/considerably/miles/way/loads/far bigger. They can all be translated with "i bhfad níos mó", even though "much" and "miles", for example, are not normally considered synonyms in English.
here are some examples of "i bhfad" that aren't being used with the comparative:
"beidh mé ar ais sula i bhfad" - "I'll be back soon (before long)"
"is glas iad na cnoic i bhfad uainn" - "far away hills are greener"
"tá tú i bhfad as marc leis an bhfreagra sin" - "you're way out with that answer