The map size has not changed since the 2013 release,
However, as you say, the online content has defo increased it. (even further on the 15th) But GTA is a bad comparison as there are not that many interiors and TBF I never bought it up.
Also, there does not seem to be any difference in file size across the 5 6 platforms.
I just thought 70GB was a tad small for such an expansive open world such as it is. Given the amount of stuff we are able to do, the different locations, the 4K textures and huge amount of dialogue I was expecting an RDR2 sized install.
Edit: Spelling and forgot Stadia.... as you do.