Do they mean people who own their own homes? If so, how could they be "forced" to move? Unless some sort of "under-occupation" rate on council tax was introduced. I don't think that would be fair - I didn't think it was fair for those in social housing to be charged for under-occupancy either. I think it is quite reasonable to have one spare bedroom.
However, some older people in large, heavy maintenance houses, might like to move to something smaller if there were attractive places available, with a garden if it was still wanted. Sometimes the cost of moving - which can be considerable and which could significantly deplete the surplus money left from the sale - would prevent people from moving so perhaps some contribution towards the cost might give the necessary incentive. If enough larger houses were made available through such a scheme, presumably that would bring the price of larger houses down and the knock-on effect would continue down the line - welcome for those hoping to buy their first home but presumably not for others. But would it be very fair for people who already have a fairly expensive asset to be able to get financial assistance to move? There are all sorts of things to consider - it's not straight forward.
I'm inclined to think there should be some sort of land tax to stop companies and individuals hanging on to land as an investment, because it is the shortage of land which fuels its ever-increasing cost and which pushes up the price of building new developments.
I also agree that some restrictions should be placed on foreign investors who just leave properties empty - there are areas of London that have been described as being like "ghost towns". If these properties were made available for rent or, if not, owners had to pay a substantial surcharge for leaving them empty, it should reduce rents across the board.