Yeah, the landowners (not always Palestinian, by the way) sold like 5% of the land to Zionists before WWII. Now how much did Israel ended up with?
Perhaps the rest of the land was just empty and had no owners? You are forgetting that the Zionists dried many, many swamps and most of the land they received originally wasn't habitable, but they worked it and made it fertile.
Also, I don't know exactly but what I do know is that about 80% of the lands Israel got was, and still is, just a big desert, the Negev.
Here's a map showing Jewish presence in Palestine from 20-48:
And here's a map showing the partition plan:
Considering all the are south of Beersheba is useless desert, that wasn't inhibited by anyone, I'd say it was a reasonable partition. I wouldn't mind giving it all to the Arabs back then, but I guess the UN thought otherwise (so it wouldn't be a 15%-85% partition of the land maybe?).
And I say this once again: Israel didn't get Palestine because of the holocaust, they Zionists didn't just move there in '45 and declared independence 3 years afterwards, they were there from 1880 and immigration never stopped since then.
You can see here:
http://www.cjpme.ca/documents/En Demographics Factsheet v.1.pdf
That by 1942 The Jews were almost 500,000 in number in Palestine, with the Arabs almost 1,000,000 in number.
Israel was declared in Palestine because this is were most Zionists were, where they wanted to form their country, and where they've built their national home for several decades.