People might have thought that, seeing that there were no windows inside the Tabernacle, there was a need for the lights of the Menorah, whereas in the Temple, which was equipped with windows (Kings I 6:4), there was no need for interior illumination. The Book of Kings makes a point of stating that the windows in the Temple were very narrow on the inside and wide on the outside, suggesting that their purpose was not to let light enter, but on the contrary, to illuminate the outside world with the spiritual light contained therein.

Solomon...made the windows narrow on the inside and wide on the outside

Our Sages (Tanchuma Behaalotcha 2) phrased it thus: when the average person builds himself a house, he arranges for the windows to be narrow on the outside and wide on the inside in order to admit the maximum amount of daylight. Solomon did the reverse. He made the windows narrow on the inside and wide on the outside in order for the Temple to radiate the maximum amount of spiritual light to the outside world. This conveyed the message that the entire Temple was a source of light, and that G‑d most certainly did not require man-made light to light up the interior for Him.

[Selected with permission from the seven-volume English edition of "The Torah Commentary of Rebbeinu Bachya" by Eliyahu Munk.]