Why would Algeria's land size have anything to do with that?

Algeria is more than 80% desert and has a population of ~46 million. Non-desert area accounts for ~480k km^2 out of their ~2400k km^2 land . Europe has far more livable space than Algeria does. Spain is pretty comparable with a population of ~48 million in 505k km^2.

I get that there are political reasons, but "Algeria is big so Algerians shouldn't need to leave" is a pretty surface-level observation.

If the French didn't want Algerians to be French they shouldn't have invaded there in the first place.

So it's like revenge? The French have to accept revenge?

I don't think the poster meant it as a revenge thing. France at some point made it illegal to deny the Frenchness of Algeria, you could go to prison for this back then. The two countries have had a very long history of close relations with its all ups and downs, and many French lived in Algeria and Algerian lived in France. I have no connection to either of those countries I'm just interested in history.