They trim back what poses the highest risk with the budget they have.
As you have identified A wider right of way costs more.
Usually for lines above some voltage, perhaps 200kV, the cost of an outage due to a tree strike outweighs the cost of additional vegetation management so they will clear the right of way wide enough that no tree can fall and hit the power line.
Around here for 130kV the right of way is still as narrow as it can be and we annually take down the riskiest trees as this is the best for our budget, which is not unlimited.
Concur, they have huge fire breaks they put high tension lines in, where the easement is easily 100' to either side of the lines. This is in Louisiana. When there's a hurricane these lines still break, but not because of trees. During heavy storms, smaller trees hit smaller (480? Some KV?) Lines that go along highways to residences. A high tension line down means a few days without power (about 3-4), a tree on a lower voltage line is usually fixed within hours.
They go through and remove damaged trees near the easements of the highway lines, as well as branches that could break into lines.
As an aside we lived on the same section of grid as the sheriff, and our power was rock solid for a few years, then he left office and now our power is better than average (at least better than our neighbors who's power line cones from the other direction).
Where I live, there are no trees in the path of high-voltage (400KV) transmission lines. Everything is cleared below them and the voltage lines are about 90-120ft high so even if some trees grow, they will not touch the lines.
As you have identified A wider right of way costs more.
Usually for lines above some voltage, perhaps 200kV, the cost of an outage due to a tree strike outweighs the cost of additional vegetation management so they will clear the right of way wide enough that no tree can fall and hit the power line.
Around here for 130kV the right of way is still as narrow as it can be and we annually take down the riskiest trees as this is the best for our budget, which is not unlimited.