I was able to resolve it. This is the sequence of actions that worked for me:
I opened the "Disk Utility" from the Launchpad. It showed RStudio-1.1.423 as one of the entries in the "Disk Images" list on the left side.
Next, I used right-click on that entry and among the list of commands in the menu, I chose the "Image from RStudio-1.1.423" command and chose the "Documents" directory as the destination folder.
It then created a new file "RStudio-1.1.423.dmg" file in the "Documents" directory:
-rw-r--r--@ 1 sdas staff 480018545 Feb 19 14:19 RStudio-1.1.423.dmg
I noted that the size of this .dmg was 480MB whereas the size of the "RStudio-1.1.423.dmg" file that I had downloaded into my "Downloads" folder was much smaller - 78.1 MB.
Next I double-clicked on the new "RStudio-1.1.423.dmg" file and then everything seemed to work as expected.
Thanks.