Welsh assembly voting: Cheryl Gillan proposes new seats for 2016 election
The next Welsh assembly election may be fought with different constituencies under plans to change the voting system.
A Wales Office consultation includes a proposal to cut the number of constituency assembly members from 40 to 30, with another 30 coming from regional lists.
Reforms would be designed to ensure "fairer votes," Mrs Gillan said.
But the Welsh government says there is no mandate for change.
It follows a planned reduction in the size of the House of Commons.
The number of Welsh MPs is due to fall from 40 to 30, breaking a link with the assembly's constituencies.
The assembly has 60 members, and at present its 40 first-past-the-post constituencies share the same boundaries as Wales' 40 parliamentary constituencies. A further 20 AMs are elected from regional lists.
Mrs Gillan unveiled a green paper which proposes either having 30 AMs occupying the same constituencies as Wales' new parliamentary seats, or redrawing the boundaries of the 40 existing constituencies so they are of equal size.
In either system the rest of the 60 seats would be filled by regional AMs, maintaining the assembly's partly proportional elections.
Mrs Gillan pointed to big differences in the number of voters in some constituencies. For example, the Cardiff South and Penarth constituency has more than 76,700 voters and Arfon has about 40,660.
"The government has come a long way since 2010 and we have been delivering some important reforms to Britain's political system which we believe will contribute to restoring trust and ensuring fairness," she said.
The status quo was not an option, she said, and although she said she had an "open mind," she rejected a proposal from the Labour Party.
Welsh Labour says the UK government does not have a mandate to make changes, but if changes are introduced it will call for 30 two-members seats, with all AMs elected in first-past-the-post elections.
Mrs Gillan said: "I have not considered that (Labour's position). I don't want to reduce the proportionality in the system."
A 30-30 Senedd chamber, with half the members representing first-past-the-post seats and half coming from the regions was "the most elegant and simple solution for the electorate, I think".
She said she had not received representations to switch responsibility for the assembly's electoral arrangements from Westminster to Cardiff Bay.
The green paper also seeks views on whether the assembly should have terms of four or five years. Terms have lasted four years since devolution in 1999, but the next election has been delayed until 2016 to avoid clashing with the 2015 general election.
A ban on candidates standing in constituencies and on regional lists could also be lifted.
The paper also raises the prospect of prohibiting "double jobbing" - stopping AMs becoming MPs or members of the House of Lords.
First Minister Carwyn Jones said: "There is no mandate for this. The electoral system for the assembly is a matter for the people of Wales and no one else.
"The Prime Minister has assured me that there would be no change to future electoral arrangements without the agreement of the assembly."
Plaid Cymru said decisions about elections in Wales "should be made in Wales and not in London".
Plaid AM Lord Elis-Thomas told the BBC Wales Sunday Politics programme that during his time as presiding officer he had been assured by Prime Minister David Cameron and Mrs Gillan that the changes to the assembly's electoral system would be considered separately to any change in the number of MPs at Westminster.
Welsh Liberal Democrat leader Kirsty Williams said her party welcomed the pledge to protect or increase the proportional element of assembly elections.
Nick Ramsay, Conservative AM for Monmouth, said the Tory group supported the current arrangements for assembly elections, in particular the element of proportional representation.
"What we certainly don't want is what the Labour Party is proposing, which is scrapping the proportional element altogether so that parties like mine and others wouldn't get their fair share of the votes.
"I think that would be a disgraceful proposal if it happened."