I think the point being made here is that the banks that would do this are not doing equity release type mortgages or small top-ups in order to just pick the larger deals which cost less administratively when they have only limited balance sheet capacity to lend.i.e. do 1 200k deal rather than 4 50k top-ups, or 2 100k deals.
Also, one could say that if the mortgage is over 200k then it is most likely on a bigger/better property so better security. It is also ruling out mortgages on almost all 1 bed apartments and most 2 beds outside Dublin.