Zevachim 62A

ולא על גבי מחילות.

and one may not build it on top of tunnels.

אמר רב יוסף, לאו היינו דתניא: ״ויכינו את המזבח על מכונתו״⁠ – שהגיעו לסוף מדותיו? והכתיב: ״הכל בכתב מיד ה׳ עלי השכיל״!

The Talmud relates that after reconsidering the reason for the expansion of the altar, Rav Yosef said: Is this not as it is taught in a baraita with regard to the verse: “And they set the altar upon its bases” (Ezra 3:3), which teaches that in the Second Temple the size of the altar reached its full measure, i.e., that it was the ideal size, whereas in the First Temple it was not the ideal size? The Talmud asks: But isn’t it written with regard to the instructions David gave Solomon about how to build the Temple: “All this in writing, as YHWH has made me wise by His hand upon me, even all the works of this pattern” (I Chronicles 28:19), indicating that the design of the First Temple was dictated by God?

אלא אמר רב יוסף: קרא אשכח ודרש ⁠– ״ויאמר דויד: זה הוא בית ה׳ האלהים, וזה מזבח לעלה לישראל״⁠ – כי בית; מה בית ששים אמה, אף מזבח ששים אמה.

Rather, Rav Yosef said: The size of the altar in the First Temple was ideal, but in the Second Temple era there was a need to expand the altar, and they found a verse and interpreted it as follows. The verse states: “Then David said: This is the House of YHWH God, and this is the altar of burnt offering for Israel” (I Chronicles 22:1). The verse juxtaposes the House, i.e., the Temple, with the altar, which indicates that the altar is like the Temple: Just as the House was 60 cubits (see I Kings 6:2), so too, the altar may be extended up to a length of 60 cubits.