Following volume not weight in the Torah measurements:[1]

All the Torah measurements are measured in volume and not weight. [Nevertheless, due to the difficulty in determining the volume of an item, therefore for simplicity purposes, we practically measure in weight.[2] Nonetheless, this weight must be commensurate to its Halachic Kezayis volume. Thus, a food which is lighter than water [of which its weight and volume is equal], and thus floats on it, will require a lesser weight in its Kezayis then a Kezayis of water. For example, if a Kezayis equals 26cc, which is equal to 26 grams of water, then a food which floats on water will need less than 26 grams for its Kezayis, and within less than 26 grams of weight it already contains a volume of 26cc. On the other hand, a food which weighs more than water, and hence sinks in it, will require a greater weight in its Kezayis, and thus will need more than 26 grams for it to attain its 26cc.[3]]  

_________________________________________________________________

[1] Admur 486:2; Rama 486:1; M”A 486:1; Maharil Seder Hagadah p. 111; Sefer Ohel Moed Hilchos Yom Kippur; Mishneh Uktzin 2:8; Rambam Pirush Hamishnayos Eidiyos 1:2; So rule regarding Issur Viheter: Peri Chadash Y.D. 99:6; Peri Toar 99:4; M”B 486:3; Kaf Hachaim Y.D. 98:5 [that one who measures by weight is feeding Treifos to the Jewish people]; Shiurei Torah Siman Alef “It is known and public knowledge from all the Poskim Rishonim and Achronim, that all the measurements of Revius, Kezayis, Kivbeitza, are to be measured in volume and not weight” and in footnote 1 “There are many proofs for this, and I have written all this in length to negate the opinion of the Kaf Hachaim 168:46 that we measure in weight and not volume, as in truth we only measure in weight those foods that had already had their volumes measured and then weighed.”; [However, see Shiureiy Torah p. 184 who writes to measure Matzah by weight; Rav SZ”A says that Rav A”C Naah actually told him to measure by volume, unlike what he wrote above to follow weight. See Seder Hearuch 79 footnote 32]; Piskeiy Teshuvos 486:1; Rav Yaakov Yosef z”l ruled to follow volume and completely negated the following of weight, thus disputing his fathers, and brothers positions.

Other opinions: Some Poskim rule that the all Torah measurements are to be measured in weight and not volume. [Maggid Mishneh Eiruvin 1:12  in name of Teshuvos Hageonim; Pesach Hadvir 130, 190 in name of a number of Poskim, brought in Shiureiy Torah 3:8 footnote 18 and Kaf Hachaim 168:46; Beis David; Mateh Yehuda 168:1; Chida; Vhemdas Yamim; Kaf Hachaim 168:46; 486:1 and 3; See Chazon Ovadia Pesach Hilchos Leil Haseder p. 42 and 73; Rav Yitzchak Yosef Shlita that his father held to always follow weight and that so he asked of him to tell the public in his Shabbos Hagadol Drashos, thus negating his brothers position to follow volume]

[2] Kaf Hachaim 168:46; 486:1 and 3 that so is the widespread custom of Jewry, and all Rabbanim, to measure in weight in order to be Yotzei according to all; Shiureiy Torah 1:1 brings that the Rambam measured a number of items in weight and that from this we can learn regarding other items to measure in weight; Shiureiy Torah 3:8 footnote 18 and page 184 writes to measure the Matzah and bread in weight, and practically gives weight measurements for all his Shiurim, after having compared them with their volume size; Chazon Ovadia Pesach Hilchos Leil Haseder p. 42 and 73; Rav Yitzchak Yosef Shlita that so was the custom of Sephardic Jewry for all generations and that his father held to always follow weight and that so he asked of him to tell the public in his Shabbos Hagadol Drashos, thus negating his brothers position to follow volume

[3] See Shiureiy Torah 3:12-13 and p. 184; Piskeiy Teshuvos 486 footnote 3