[OSM-talk-ie] Reduction of size by area of exported OSM or GeoJson image

[Brian Hollinshead]

I am able to take an OSM or GeoJson file of County Carlow boundary and use
in leaflet or UMap.
I now would like a file for Carlow which is only 80% the area but the same
shape to overlay to show the population decrease 1841 to 1851 and having
the same lat/lon centre.

I have QGIS but am not familiar with it, any suggestions most welcome,
thank you.