"He shall pay double" (Exodus 22:3): It has been taught: Rabbi Jacob says: "He shall pay double" - even without an oath. Or perhaps it means only with an oath? You may answer: it was not so. For let the Merciful One not write "he shall pay double" concerning the thief, and it would follow by an a fortiori argument from one who falsely claims that the deposit was stolen [by another]. If concerning one in whose hand the object came lawfully, the verse says he pays double, then the thief himself, in whose hand the object came unlawfully, all the more so. So why then did the Merciful One write "he shall pay double" for the thief? To teach that it applies even without an oath. His disciples asked Rabban Yochanan ben Zakkai: Why did the Torah treat the thief more severely than the robber? He said to them: The robber equated the honor of the servant with the honor of his Master; but the thief did not equate the honor of the servant with the honor of his Master. As it were, the thief acted as though the Eye above does not see and the Ear above does not hear, as it is said, "Woe to those who dig deep to hide their counsel from the LORD" (Isaiah 29:15). Rabbi Meir said: They told a parable. To what may this be compared? To two men in a city who made a feast. One invited the townspeople but did not invite the king's household; and one invited neither the townspeople nor the king's household. Which of them deserves the greater punishment? The one who invited the townspeople but did not invite the king's household.
Why the Torah Treats the Thief Harsher Than the Robber
Yalkut Shimoni on Torah 343:2
שְׁנַיִם יְשַׁלֵּם, תַּנְיָא, רַבִּי יַעֲקֹב אוֹמֵר, שְׁנַיִם יְשַׁלֵּם שֶׁלֹּא בִּשְׁבֻעָה. אוֹ אֵינוֹ אֶלָּא בִּשְׁבֻעָה, אֲמַרְתְּ לֹא כָּךְ הָיָה, דְּלֹא נִכְתּוּב רַחֲמָנָא שְׁנַיִם יְשַׁלֵּם בְּגַנָּב, וְתֵיתֵי בְּקַל וָחֹמֶר מִטּוֹעֵן טַעֲנַת גַּנָּב, דִּבְהֶתֵּרָא אָתָא לְיָדֵיהּ, אֲמַר קְרָא לְשַׁלֵּם תְּרֵי, גַּנָּב עַצְמוֹ דִּבְאִסּוּרָא אָתָא לְיָדֵיהּ לֹא כָּל שֶׁכֵּן. אֶלָּא שְׁנַיִם יְשַׁלֵּם דְּכְתַב רַחֲמָנָא לָמָּה לִי, וַאֲפִלּוּ שֶׁלֹּא בִּשְׁבֻעָה. שָׁאֲלוּ תַּלְמִידָיו [אֶת רַבָּן יוֹחָנָן בֶּן זַכַּאי], מִפְּנֵי מָה הֶחְמִירָה תּוֹרָה בְּגַנָּב יוֹתֵר מִבְּגַזְלָן, אָמַר לָהֶם, זֶה הִשְׁוָה כְּבוֹד עֶבֶד לִכְבוֹד קוֹנוֹ, וְזֶה לֹא הִשְׁוָה כְּבוֹד עֶבֶד לִכְבוֹד קוֹנוֹ, כִּבְיָכוֹל עָשָׂה עַיִן שֶׁל מַעְלָה כְּאִלּוּ אֵינָהּ רוֹאָה וְאֹזֶן שֶׁל מַעְלָה כְּאִלּוּ אֵינָהּ שׁוֹמַעַת, שֶׁנֶּאֱמַר (ישעיה כט, טו) "הוֹי הַמַּעֲמִיקִים מֵה' לַסְתִּר עֵצָה" וְגוֹ'. אָמַר רַבִּי מֵאִיר, מָשְׁלוּ מָשָׁל לְמָה הַדָּבָר דּוֹמֶה, לִשְׁנֵי בְּנֵי אָדָם שֶׁהָיוּ בָּעִיר וְעָשׂוּ מִשְׁתֶּה. אֶחָד זִמֵּן אֶת בְּנֵי הָעִיר וְלֹא זִמֵּן אֶת בְּנֵי הַמֶּלֶךְ, וְאֶחָד לֹא זִמֵּן לֹא בְּנֵי הָעִיר וְלֹא בְּנֵי הַמֶּלֶךְ. אֵיזֶהוּ עָנְשׁוֹ מְרֻבֶּה, זֶה שֶׁזִּמֵּן בְּנֵי הָעִיר וְלֹא זִמֵּן אֶת בְּנֵי הַמֶּלֶךְ.