If you have done a clean install using the setup from www.swtor.com/download the streaming launcher is default and should offer the minimum download.
If using the non streaming as per this link the entire game needs to be downloaded first.
February 2018 - last edited February 2018
The only difference between the Streaming and Non streaming is just how the launcher patches, game size will remain at approx 40 GB in size, the 30 Gb answer in this thread was 2 years ago
You need a lot more space than the game will end up at. About halfway downloaded, it was 64GB on my drive. It then reduced to 44GB. They should inform people more accurately because I ran out of space the first attempt because the installer said it needed only 24GB.
My guess is that it downloads, decompresses files, then removes the archives, thus the reason for the fluctuations. Download is slow too. I have 1GB Internet and it's been running for hours.