I am going to compare a 50kg and a 1g spider with a silk radius of .003mm. The breaking stess would be about 1000MPa for an average spider. Lets say the giant spider manages to get this up to 2000MPa which is the strongest in the world.
Quick calculation shows that the 1g spider can hold about 3 times its body weight of a single thread of silk. How much would that need to be for a 50kg spider? Only .5mm
This calculation is enough to show that in theory a spider with thin strands (say 2-3 mm) would be possible.
In practice a spider often uses several strands of silk together. I have my doubts about the accuracy of the .003mm radius. Basically, however, this shows that a giant spider could still possibly use silk like a small one iff it were worth it. I don't believe that it is an evolutionarily favorable path which is why we don't see it. In your world, however, you could possibly make it work.
Please note that there are likely many nuances that would make this more difficult for a larger spider. I don't want to lift myself by such a small part of my body. The thin diameter likely increases mechanical strength more than bulk properties would imply. There is likely evaporation which occurs in the thin strands which would not occur in a thicker one. There is potential to focus stress on a small region of the silk. Nevertheless, spider silk's properties are incredible and, while they would need to be thicker for a giant spider, they would not really need to be THAT thick.