I did a report on this back in University. This was actually Pilsudki's plan during the 1919 War. Didn't really work out. The Poland of today is really odd because it's one of the MOST homogenous countries in Europe, when in the past it had always been a collection of many different poeples.
The Poland shown on the map (the darker area) is a lot more homogeneous than interwar Poland actually was, as like you say there were large Belarussian and Ukrainian minorities and smaller Lithuanian and German minorities.
The claimed areas on the map which actually have a Polish majority are Upper Silesia, southern East Prussia and parts of the Vilnius region and western Galicia.