The monuments under review were designated by four presidents over the last two decades. Several are about the size of the state of Delaware, including Mojave Trails in California, Grand-Staircase Escalante in Utah and Bears Ears, which is on sacred tribal land.
No other president has tried to eliminate a monument, but some have trimmed and redrawn boundaries 18 times, according to the National Park Service.
Zinke told The Associated Press last month that unspecified boundary adjustments for some monuments designated over the past four decades will be included in the recommendations submitted to Trump. None of the sites would revert to new ownership, he said, while public access for uses such as hunting, fishing or grazing would be maintained or restored.
He also spoke of protecting tribal interests and historical land grants, pointing to monuments in New Mexico, where Hispanic ranchers have opposed two monuments proclaimed by Obama.
Zinke declined to say whether portions of the monuments would be opened up to oil and gas drilling, mining, logging and other industries for which Trump has advocated. It was not clear from the memo how much energy development would be allowed on the sites recommended for changes, although the memo cites increased public access as a key goal.
A spokeswoman for Zinke referred questions Sunday night to the White House, which did not offer immediate comment.
If Trump adopts the recommendations, it would quiet some of the worst fears of his opponents, who warned that vast public lands and marine areas could be lost to states or private interests.
But significant reductions in the size of the monuments, especially those created by Obama, would mark the latest in a string of actions where Trump has sought to erode his Democratic predecessor's legacy.
The recommendations cap an unprecedented four-month review based on Trump's claim that the century-old Antiquities Act had been misused by past presidents to create oversized monuments that hinder energy development, grazing and other uses.
The review raised alarm among conservationists who said protections could be lost for areas that are home to ancient cliff dwellings, towering sequoia trees, deep canyons and ocean habitats. They've vowed to file lawsuits if Trump attempts any changes that would reduce the size of monuments or rescind their designations.
Zinke had previously announced that no changes would be made at six national monuments — in Montana, Colorado, Idaho, California, Arizona and Washington. He also said that Bears Ears monument in Utah should be downsized.
In addition to shrinking six monuments, Zinke recommends changes at several other sites, including two national monuments in New Mexico: Organ Mountains-Desert Peaks and Rio Grande del Norte.
He also recommended changes to Katahdin Woods and Waters in Maine.
Jamie Williams, president of the Wilderness Society, said the recommendations apparently made by Zinke "represent an unprecedented assault on our parks and public lands" by the Trump administration.
"This callous proposal will needlessly punish local, predominantly rural communities that depend on parks and public lands for outdoor recreation, sustainable jobs and economic growth," Williams said in a statement.
"We believe the Trump administration has no legal authority to alter or erase protections for national treasures. If President Trump acts in support of these recommendations, The Wilderness Society will move swiftly to challenge those actions in court," Williams said.