The size column in the Environment pane grid view shows zero size for some data frames, but not others. For instance, try:
test1 <- iris test2 <- mtcars
test1 has 0 B in the size column, whereas test2 has 7 KB.
This is not a function of how big the data frame is. I have just had a dataframe of 1 GB show as 0 B size!
RStudio Version 1.2.5033