The significance of one-third in the book of the Revelation is that certain plagues / judgments start with the warning that one-quarter (of this, that and the other) will be destroyed / taken down, then further on, that is upped to one-third, and by the final 7 plagues, everyone on the earth with the mark of the beast is afflicted. There is a numeric progression: from one-quarter, to one-third, to all.
This means that your second question, "Why is everything 1/3 in Revelation?" is incorrect. NOT everything is one-third. The proportions affected start with one-quarter. When the plagues are ratcheted up, that becomes one-third. Then just before the last trump, it's everyone and everything still under Satan's invisible sway.
Start with Revelation 6:7. You already have the verses mentioning one-third, then read Rev. 13:8 & 16 and 16:2-4.
The significance of one-quarter is mentioned in this book, after dealing with the riders on the red horse, the black horse, and the pale horse-rider called Death, with Hell (Hades) closely behind. They are given authority to kill one-quarter of the people on earth, Rev. 6:3-8:
"The Lamb opens the seal, and these canter forward into time on their grisly errand over all the earth, with power given to them from Almighty God to kill the fourth part thereof, again and again, generation by generation. Death effects this and by four means, for the earth, the whole earth, is that from which the fourth part is reaped by the grim reaper, the meanwhile Hell drawing the souls into its habitations until the resurrection at the Last Day... By these things a fourth part of the earth is taken by the direction
of the rider on the pale horse, with Hell embracing the souls of the
lifeless corpses. The cause is from above; only the means is from
below. But fallen man sees nothing but the means... It is a fourth
part out of the population, and three quarters of today's people
barely notice their passing. Barely notice. Nor yet at all do they
hear the hoof-beats of the pale horse at their own heels." (The
Revelation of Jesus Christ pp 161-3, John Metcalfe)
Once the significance of a quarter is seen, then when that is upped to a third, it means even more dreadful results from more plagues from heaven are felt on earth. Yet when the plagues are ratcheted up, the only result is that those afflicted by them refuse to repent of their sins! Indeed, they curse God all the more-so! And then they wonder why the final plagues result in everyone being destroyed? Read Rev. 9:18-21 for the reaction of the one-third, then 16:9-11 & 21 for everyone's reaction just at the end. That's the significance. God's judgments make the right-hearted repent and turn to him, but impenitent sinners just curse God even more than before when they experience his judgments! This means God is utterly justified for finally crushing all such ones along with Satan and his hordes - Rev. 19:11-21.