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, an extra fifth. 1
Rashbam: It teaches us that if a Zar eats T'rumas Ma'aser, he transgresses the La'av of "ve'Chol Zar Lo Yochal Kodesh". 2
Rashi: Like Terumah Gedolah, which is called 'Reishis Dagan min ha'Goren'. From the granary, because Terumos and Ma'asros of grain are taken only after the flattening of the pile, which follows the winnowing (Rashi in Bava Metzi'a, 90a).
Seforno: And 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.
And what is "ka'Mele'ah min ha'Yekev" referring to?
Rashi: "Mele'ah" means 'after they are ripe'
In which regard is the Torah comparing T'rumas Ma'aser to Terumah Gedolah?
Torah Temimah (citing Gitin, 30b): To teach us a. that, just as Terumah Gedolah 1 is taken by assessment, 2 via thought alone and generously, so too T'rumas Ma'aser, and b. that just as the Yisrael has permission to separate Terumah Gedolah, so too, is he permitted to separate T'rumas Ma'aser.