This rule is targeted at nations large enough that any mainland claim would be tiny in comparison with their island territories. If your entire country is based in an large archipelago, it makes little sense you should have a small piece of land in the mainland. It does make sense a nation with small islands to have a larger portion of its territory in the mainland.
The size limit is there because there will be future nations coming to claim their land, and we need to ensure there is enough space in the map for them. The current limit allows nations to be as large as around 1 200 000 km^2, depending on their specific location. That is a considerably large nation, but one also should consider that larger is not always better. What matters is not how many millions of squared kilometres your nation has, but how you develop it.
I should point out that the limit is not absolute. If there is a new user who comes with an interesting nation concept that would necessarily call for a slightly larger nation, it is entirely possible to let them claim a territory larger than 60 000 px.
I hope this helps clarify the reasoning behind both rules.