Actual size is 66GB as displayed on the bottom but on top the size is showing as 128TB.
Probably a Dolphin bug. Mine shows 128,5TiB for a 300GiB drive (141GiB used). Using ext4. So I think nothing to do with btrfs
I am seeing the same thing here. I am guessing that you’ve got an SSD ─ I for one do, and it’s a 1 TB Samsung drive ─ and that 128 TiB is the total capacity with the overprovision for wear-leveling included. 
I didn’t know about overprovisioning, but from what I’m reading it’s only a percentage of the declared size (5 - 30%). This is way over.
No, overprovisioning is huge. A modern 1 TB SSD can actually store 400 TB. But I’m not excluding that it could just as easily be a bug in Dolphin, in this case.
I’m sorry to say that that is hard to believe. Do you have some link to read about what you are claiming?
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I did, yes, but I don’t have it anymore now. But do keep in mind that it cannot store 400 TB as a whole. Its capacity for direct storage is 1 TB. The 400 TB is what it can handle in terms of write/erase capacity.
Ah, you are talking then about lifespan of a SSD drive. But that’s different than overprovisioning.
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Not all that different, considering the limited rewritability of flash memory. 
Yes, they are related but they are different things. Over-provisioning is reserving a percentage of the drive to help with performance and durability of the drive (due to how NAND flash memory works). But you can not seriously say that they are the same (or not so different). It’s like velocity and distance. Sure they are related but it’s not the same 
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I get the same indication when i click the “Calculate” button under size on top, and i don’t use any SSD.
What i think is actually causing it is:
It’s the max value used in the variable to display that size which does not get updated properly.
Because the number between the ( and ) are changing while it calculates, but not the number before that…
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I’m not using a SSD
Reported on KDE Bug tracking system