The solution is done in another thread about MRNet : Stanford MRNet Competition trouble downloading dataset to gcp - #3 by kushaj (I guess it is what @nswitanek used, no?):
- Use this Google extension to download in GCP (wget ...) the 6 Go MRNet dataset: it works well 
The structure after unzip is the following one:
- train
-- axial  
-- coronal  
-- sagittal
- train-abnormal.csv
- train-acl.csv
- train-meniscus.csv
- valid
-- axial  
-- coronal  
-- sagittal
- valid-abnormal.csv
- valid-acl.csv
- valid-meniscus.csv
With the command du -h MRNet-v1.0 in my terminal ubuntu, I get the following folder sizes:
258M    MRNet-v1.0/valid/axial
222M    MRNet-v1.0/valid/coronal
230M    MRNet-v1.0/valid/sagittal
709M    MRNet-v1.0/valid
2.4G    MRNet-v1.0/train/axial
2.1G    MRNet-v1.0/train/coronal
2.2G    MRNet-v1.0/train/sagittal
6.6G    MRNet-v1.0/train
7.3G    MRNet-v1.0