There are several reasons why this election is unusual. The proportion of votes obtained by the largest party isn't one, though. 28% for LREM (32% if you include its ally Modem) is actually on the low side.
An unusually high number of seats
France doesn't have proportional representation, so the propotion of votes obtained by a party isn't a good indication of how many seats a party or coalition gets. Members of Parliament are elected according to a two-round, first-past-the-post system. In each constituency, the top two candidates from the first round compete in the second round, and the one that gets the most votes in the second round gets elected. (It's possible to have more candidates in the second round but that requires them to pass a voter threshold and turnout was so low that this only happened in a single constituency this time.) As in any election where candidates compete for individual seats, majorities get amplifies: winning a seat with 50.01% of the vote is just as good as winning with 99.99% of the vote.
The two-round system and the winning coalition's political positioning amplifies the winning coalition's victory even more. Macron's party has a centrist positioning, in a country where the traditional division is left vs right. If a centrist candidate makes it to the second round, and the opposition is a left-wing candidate, the centrist candidate tends to get votes from right-wing voters. Likewise if the opponent is a right-wing candidate. So even in many places where the centrist candidate came second in the first round, they'll win the second round from fallback votes. This is why Macron is poised to get a very comfortable majority.
It's likely that LREM (Macron's party) will get a record number of seats in Parliament, although in all likelihood as I write it won't be a record for winning coalition (in 1993) the right-wing coalition got 472 representatives).
An unusual political positioning
A second reason why this election is unusual is the nature of the winner. Ever since the current electoral system was established in 1958, French politics has been dominated by a left/right opposition, with the right mostly dominated by a socially conservative movement (roughly comparable to the US Republicans and the British Conservatives) and the left consisting of a spectrum from communists (now moribund) to social-democrats (roughly comparable to the British Labour before New Labour). Although there have always been socially progressive, economically liberal movements (roughly comparable to the British Liberal-Democrats) that label themselves “centrist”, they have mostly allied themselves with conservatives to form a right-wing block. Macron's party is a centrist movement that won by itself rather than in a coalition with the right.
In previous elections, centrist candidates who didn't particularly appeal to the mainstream right tended to be beaten by both left-wing and more right-wing candidates. In this election, most centrist candidates made it to the second round, for a variety of reasons. The main reason is that freshly elected president always get a popularity boost (“state of grace”). Another reason is that the left went into this election divided, and the right ran a poor campaign (especially in the presidential election just before), so there weren't many constituencies where both a right-wing candidate and a left-wing candidate managed to qualify for the second round.
A new party
Macron's party LREM is barely one year old. And it isn't a mere renaming of an older party, either. The closest pre-existing party with a somewhat similar political positioning was MoDem, which got 7.6% of the vote in the first legislative election after it was founded in 2007 and had been losing popularity ever since. Nor was LREM a spin-off of another party: Macro had been briefly a member of the Socialist Party and an advisor and minister of a Socialist Party president, but his political positioning was initially considered too far right for even many “New Labour” supporters.
A party came from basically nowhere to dominating the political scene, on a course that another had tried before and failed. That's a third reason why its victory is unusual.