For a pure stock device with a locked bootloader, the only upgrade path is via OTA file bundle if you insist on keeping the bootloader locked. The large (complete) factory images can only be used with fastboot and an unlocked bootloader.
To do the OTA manually, rather than via automatic WiFi download , I recall that the stock recovery no longer has a "update.zip" menu choice, but instead has a menu choice for ADB sideload*. This implies that you need to get ADB working ahead of time to manually apply the OTA.
You are correct about the two file types - the smaller one is a differential patch set (OTA), and the larger a complete factory ROM for use with fastboot as mentioned above.
Because the OTAs are patch sets
they are tightly matched to the version of software on your tablet... and you said something vague about your current software version. For instance, you mentioned the OTA file
... but said that you have "4.1"
4.1.2 would be JZO54K, not JOP40D (which is 4.2.1; JDQ39 is 4.2.2)
So, please state your exact software version number you currently have - and the version you want to go to. And have a look at the OTA thread
*might depend on which version of stock software you have on the tablet.