As usual, Mefarshim quoted are from here:
Part 1: Were the Jews actually greater in number than the Egyptians?
Ralbag, Chizkuni, R"A Ben Harambam and others explain that obviously the Jewish population had not grown larger than the population of the entire Egypt, rather, they were growing at a greater rate than the Egyptians, who feared that at some point the Jews would eventually outnumber them. This approach can be read into the Passuk (1:9) וַיֹּ֖אמֶר אֶל־עַמּ֑וֹ הִנֵּ֗ה עַ֚ם בְּנֵ֣י יִשְׂרָאֵ֔ל רַ֥ב וְעָצ֖וּם מִמֶּֽנּוּ in one of two ways - either "they will soon be greater and stronger than us" or "they are multiplying and strengthening faster than us" (see Mefarshim).
Others (RDZ Hoffman, see also Nechama Leibowitz) suggest that Pharaoh simply lied to the nation in order to convince them. Daat Mikra (Shemos 1:9, fn16) notes a number of parallels including Bereishis 26:26, Bamidbar 22:6.
There are, however, some commentaries and Targumin that (possibly) understand Pharaoh's comment to be literal. The most significant and clearly stated would be Ramban 1:10, who gives a number of reasons why Pharaoh did not attack Benei Yisrael directly, the third of which is that the Jews were as strong as the Egyptians at that point. According to those Mefarshim, we must see how the Egyptians enslaved them.
Part 2: How did the Egyptians enslave them?
Ramban, based on the above, follows many Mefarshim in explaining that the Egyptians tricked them into working for them as "taxes" (see 1:11), which began to slowly weaken them, and the Egyptians were eventually able to kill the Jewish male children as well. There are a number of adaptations of this understanding within the other commentaries, but almost all of them work within a similar framework.
The famous Chazal on this is found in Sotah 11b, where the word "Befarech" (1:13) is explained as "Befeh Rach", with soft words, that the Egyptians convinced them and paid them to work originally (Rashi there in above link). This is quoted by Chizkuni 5:4, Daas Zekeinim 1:11 and 5:4, and many others.
In conclusion, even according to those Mefarshim who felt that the Jewish nation were strong enough to take on the Egyptians before the enslavement occured, once they were already enslaved, they were certainly too weak and broken to fight back, as evidenced by the rest of the Parsha and the commentaries brought above.