Did Zionists “take over” 73% of their land from Palestinians?
"Take over" is a bit vague, and subject to interpretation. The situation is muddled enough that it is not really valid to say "yes" or "no" to that objectively.
As the question currently focuses on 1948, let's look at the immediate history lead up to, and following that period.
In 1947, the United Nations passed Resolution 181, which outlined the establishment of a legally recognized Independant Arab State as well as the state of Israel.
The Palestinian Arabs refused to acknowledge or comply with Resolution 181:
p. 396 The immediate trigger of the 1948 War was the November 1947 UN partition resolution. … The Palestinian Arabs, along with the rest of the Arab world, said a flat “no”… The Arabs refused to accept the establishment of a Jewish state in any part of Palestine. And, consistently with that “no,” the Palestinian Arabs, in November–December 1947, and the Arab states in May 1948, launched hostilities to scupper the resolution’s implementation.
Benny Morris (2008). 1948: a history of the first Arab-Israeli war
The land Israel declared the state of Israel territory was allocated to them by a UN-passed agreement. Arguably, they were given the initial land, rather than taking it from the Palestinians (and since the Palestinian Arabs rejected the resolution that would grant them their own recognized independent state, the Palestinian independent state did not come into existence).
After the Arab-Israeli war, the armistice agreements increased the volume of land held by Israel, but did not give them ownership. Again, "take over" is vague and arguable in this case.
In 1948 they took over 73% of it.
The division that was supposed to occur according to Resolution 181 would have resulted in the Jewish State comprising roughly 5,500 square miles, or about 56 percent of Palestine. The Arab State was to be 4,500 square miles. Source.
While that source could be argued as biased, anti-zionist sources agree with the division being roughly 56% allocated to the Israelis, and 44% allocated to the Arabs.
In 1949, Israel wound up with control over a significantly larger area than was outlined in the 1947 UN plan, but that was the result of the armistice agreements signed by the aggressor Arab states who had attacked Israel on the day it was formed. Israel did not own this additional land, as the agreements were quite specific that the borders were temporary.
The armistice agreements were intended to serve only as interim agreements until replaced by permanent peace treaties, but no peace treaties were actually signed until decades later.
Did Israel own the lands it occupied in 1948, according to international law?
No, not really. However, the concept of international law simply did not apply to the situation in 1948.
International law applies between nations and states, and the participation of members is generally consensual. Consent is typically provided through the signing of treaties, and these treaties grant rights to International Courts to provide rulings in areas of dispute. There are numerous international bodies created by treaties adjudicating on legal issues where they may have jurisdiction. The only one claiming universal jurisdiction is the United Nations Security Council.
The UK had terminated their official oversight of the area by declaring the end of Mandatory Palestone on May 14, 1948, and therefore no international laws applying to the UK were applicable from that date, unless a new governing state or nation claiming control of part or all of the region were to sign a reciprocal treaty.
As the newly-founded state of Israel had signed no treaties with other nations regarding land-ownership.
Similarly, the Palestinian Arabs had signed no such treaties.
So, at this point, no international law applied.
Israel was recognized as a member state of the United Nations on May 11, 1949.
However, even then the legal territory was not defined. In September of 1949, the United Nations Concilliation Committee for Palestine put forth a number of recommendations regarding the establishment of a permanent regime in the area:
The Commission has drawn up a plan which, in its opinion,can be applied in the present circumstances. This should not, however, be interpreted as in any way prejudging the final settlement of the territorial question in Palestine. It is the considered opinion of the Commission that the provisions of the proposed Instrument are sufficiently flexible to make it possible for the Instrument, with certain modifications, to be applied to any territorial situation that might emerge from the final settlement of the Palestine problem, and that it can be adopted by the General Assembly at its forthcoming session if the Assembly thinks fit.
In view of the fact that the question of the demarcation line between the Arab and Jewish zones of the area of Jerusalem (article 2) is intimately connected with the final settlement of the Palestine problem, the Commission has not deemed it advisable for the present to make any proposal as to the actual demarcation line. The Commission believes that the Instrument can be put into effect with the present armistice line as a provisional demarcation line, without prejudice to the establishment of a definitive line at a later stage.
So, to summarize:
No, the Jews did not take 73% of their land from the Palestinians. No, they did not legally own the land they occupied in the time period, but neither did the Palestinian Arabs or anyone else, as they were disputed territories not yet covered by international treaties or laws.