What exactly is the Torah saying here?
Rashi: The Torah is teaching us here that the T'rumas Ma'aser that the Levi gives to the Kohen is forbidden to Zarim and to Teme'im like Terumah Gedolah, and that if a Zar eats it be'Meizid, he is subject to Misah (bi'Yedei Shamayim); be'Shogeg, he pays an extra fifth. 1
Rashbam: It teaches us that if a Zar eats T'rumas Ma'aser, he transgresses the Lav of "ve'Chol Zar Lo Yochal Kodesh". 2
Rashi: Like Terumah Gedolah, which is called 'Reishis Dagan min ha'Goren'. From the granary, because T'rumos and Ma'aseros of grain are taken only after the flattening of the pile, which follows the winnowing - Rashi in Bava Metzi'a, 90a.
Seforno: Whatever has been measured, counted or weighed is generally not subject to B'rachah - See Ta'anis, 8b.
Just as Terumah Gedolah bestows a B'rachah upon the Chulin that remains in the hands of the Yisrael.
What is "ve'cha'Mel'eah min ha'Yekev" referring to?
In which regard is the Torah comparing T'rumas Ma'aser to Terumah Gedolah?
Torah Temimah (citing Gitin, 30b): To teach us that a. just as Terumah Gedolah 1 is taken by assessment, 2 via thought alone, so too Terumas Ma'aser, and b. just as the Yisrael may separate Terumah Gedolah, so too, may he separate T'rumas Ma'aser.