היצוע [הַיָּצִ֨יעַ] הַתַּחְתֹּנָ֜ה חָמֵ֧שׁ בָּאַמָּ֣ה רָחְבָּ֗הּ וְהַתִּֽיכֹנָה֙ שֵׁ֤שׁ בָּֽאַמָּה֙ רָחְבָּ֔הּ וְהַ֨שְּׁלִישִׁ֔ית שֶׁ֥בַע בָּאַמָּ֖ה רָחְבָּ֑הּ כִּ֡י מִגְרָעוֹת֩ נָתַ֨ן לַבַּ֤יִת סָבִיב֙ ח֔וּצָה לְבִלְתִּ֖י אֲחֹ֥ז בְּקִֽירוֹת־הַבָּֽיִת׃
最底層的側屋寬五肘,中層寬六肘,第三層寬七肘;因為他在殿外四圍的牆上造了收進的凹位,使樑木不必插入殿牆。
Rashi
The bottom chamber. One each side there were three [tiers of] chambers, one on the other. The measurement of its length is not stated here, but in Maseches Midos14 4:3. we learned that there were thirty-eight chambers, fifteen in the north, five on top of five and five on top of them, and the same was in the south. And eight in the west, three on top of three and two on top of them. The bottom ones were five [amohs wide] and the middle ones six, and the top ones seven. Why? Because he made depressions in the wall of the house round about on the outside. The height of the chamber was five amohs, and as the height of the wall of the Sanctuary reached five amohs, he decreased its thickness and recessed it one amoh inward, and on that recess he placed the ends of the beams of the roof at the top of the chamber which served as the floor of the middle one. And as the wall reached [the height of] ten [amohs], he recessed it [another] amoh to place the ends of the beams of the roof of the middle [chamber] which served as the floor of the upper [chamber]. Thus we find the middle [chamber] is wider than the lower one by one amoh, and the upper [chamber] is wider than the middle one by one amoh.
Rashi
So that [the beams] would not be attached to the walls of the Beis Hamikdosh. In order that the ends of the beams do not attach [to the walls] through perforations in the walls of the house, since this deteriorates the wall and it diminishes its beauty.