Or HaChaim on Numbers 28:9/אור החיים על במדבר כ״ח:ט׳

1 וביום השבת שני כבשים, "And on the Sabbath day, two sheep, etc." The reason that the additional communal offering on the Sabbath was smaller than any of the parallel offerings on the festivals and even the intermediate days of the festivals is the mystical dimension of לחם משנה, the two loaves we eat on the Sabbath at each meal. The two additional sheep offered on the Sabbath symbolise the idea which has been eternalised in the custom of the two loaves, themselves a reminder of how G'd provided a double portion of manna to the Israelites in the desert each Friday. The mystical element of the number seven is alluded to by the fact that only this day is also called "the seventh day." Even though some festivals have a seventh day, the significance of the seventh day of a festival does not have anything in common with the mystical dimension of the seventh day which occurs regularly every week.