Why did they say "Ki Atzamta Mimenu Me'od"?
Seforno: Because they were afraid that his wealth would lead him to rise against them. 1
Ohr ha'Chayim: The kingdom's greatness was not recognizable in the face of Yitzchak's greatness (refer to 26:13:1:1). This is the ultimate disgrace of the king!
Riva: They thought that he had the best land, for it produced so much. After he left, and the land produced like others, they realized that it was because HaSh-m was with him.
Oznayim la'Torah: 'Even if there is only one Jew in the country, there is no room for him - in spite of the fact his wealth was the result of his own hard work and did not come at their expense, and even though they could have learned from him the art of farming!' 2
Sechel Tov: Avimelech said so. Although he said "Mimenu" - in the plural - he means 'more than me.' Oznayim la'Torah - Kings often speak in the royal 'we.'
Yalkut Shimoni (111): All your wealth is Mimenu, i.e. from us. Malbim - they thought that he is taking away the abundance that is fit for themselves. They did not realize that a Tzadik is like a person's heart - it takes life (blood) from all the limbs, and returns life to them, and without it, the body could not exist for a moment.
From where did they demand that he leave, and why?
Ha'amek Davar: He must leave the big city where the nobles and rich residents live. The king could not bear the pain of his people. He was forced to virtually annul the Bris with Avraham, namely that Avraham's seed could live in Avimelech's land where they wanted, like residents. This was a sign for the future, that Nochrim will limit where Jews may live, claiming that they take from the country's income.