Equal sized files - why the differences?

[Pepinillo]

First time user.
I'm just testing the FFS comparing 2 folders located on my desktop. The two folders contain the same pictures but with different names (I changed the names just to check how the FFS works).
When I use the "Comparison by File size", the two pictures seem to be the only ones in both folders. I mean, the FFS can't identify that they have the same file size and it treats them as different files.
Any idea of what's happening?

[Plerry]

FreeFileSync (FFS), as the name suggests, is (primarily) synchronization software, and is not a duplicate finder.
When Comparing, FFS only compares identically named files in identical (sub)folder locations relative to the left and right base location.
FFS has detected that the two files shown on the left do not exist in the right location, and intends to copy those two files left-to-right. Similarly, FFS has detected that two files exist in the right location that do not exist in the left, and (because you apparently chose the Mirror sync variant) FFS intends to delete the two files shown on the right.
Not being a duplicate finder, FFS is blissfully unaware that there files on the left and right that have the same size (and possibly even same content), as FFS does not look for that because the files are named differently.

[Pepinillo]

Then, what is the "File size comparison" made for?
And, do you know other software to compare that files properly?

[Plerry]

If identically named files in identical (sub)folder locations relative to the left and right base location exist, FFS will compare those two to see if it needs to take (sync) action.
This comparison can be based on file sizes, file dates and/or file content, depending on what you selected.
If FFS detects an inequality between said left and right file based on the selected comparison criterium, it will propose sync actions to correct that inequality, according to the rules applicable for the selected FFS sync variant (or your custom rules).

> do you know other software to compare that files properly
Well, FFS compares "properly" for its intended purpose.
But that apparently does not suit your needs.
Your needs seem to be for a duplicate finder or something close to that.
Simply Google for Duplicate Finder.

[Pepinillo]

As you see in the pictures I uploaded, the comparison I did is based on file sizes, and with this criteria FFS is not identifying the similarity. I've tried the same locating the pictures in a subfolder called "X" in each Folder (Folder A and Folder B) and the result is the same.
If I'm comparing with the "File sizes" criteria, there shouldn't be an inequality, why is there?

[Plerry]

Your left-side file is named DSC_0008.JPG, while your right-side file of identical size is named Guapi.JPG, so your left- and right-side files are not identically named, and as explained before, FFS will not even compare them (even if these files reside in the identical (sub)folder relative to the left and right base location).
The same holds for the other left- and right-side file.
Re-read my previous replies in that realization ...

[Pepinillo]

First of all, thank you for your attention.
I think I understood you correctly. The issue is not the name. I know the names are different, and that's why I'm comparing based on the "file size" instead of the "file dates and size". If FFS is checking just the file size (and not the name) I can't understand why it find them different.
Excuse my insistence.

[Plerry]

At the risk of repeating myself:
FFS will only compare identically named files and will never compare non-identically named files, irrespective of the compare method that is selected.
And that makes perfect sense for synchronization purposes.

[Pepinillo]

Finally, I got it. Probably, it was a translation mistake. Thank you.