Hear me out. I think there is considerable evidence to suggest that Thargoid Surface Sites are in fact Thargoid Titans.

First, there is this image by CMDR Spiridon on this forum post that compares both Thargoid Surface Sites (which will be referred to as TSS) and a regular Thargoid ship. Both are very similar. You can even make the case that it looks like a bigger Thargoid ship. Considering that Thargoid ships have the same similar structure, although Hydra and Scout deviate a little bit from this main pattern, although not entirely, it's very safe to say that, this octagon-ish pattern is common throughout the entirety of the Thargoid fleet. Which means that this would also apply to Titans.

Now let's link TSSs and Titans, if you aren't convinced.

According to this this whoopsie daisy from Frontier basically confirming that we should have seen Thargoid Titans already, Rogue Signal Sources should contain a Titan inside. FSS scans provide audio from sources so you can hear battles happening or ships moving from afar. Pointing it to the Rogue signal source causes us to be able to hear it directly, as demonstrated by this galnet news digest from a few days ago.

TL;DR: we have a ground structure which has the shape of a Thargoid ship, which has interior that makes simiar noises to Rogue Signal Sources recently found in space.

Piecing this all together, I guess it's very much likely, not to say certain, that Titans look like, resemble, or are, TSSs. I'd say that's quite obvious by now. Let's hope the TSSs are decomissioned and won't go flying tomorrow with 4.0. And let's hope, even more, that the Titan where the Proteus Wave was deployed don't go flying up as well.

Be safe, brace for impact.