Memory units in mem_used() after mathemematical operation

[Alectoria]

I want to convert units in mem_used() but I am confused by (1) how units are switched automatically and by (2) how units are changed but not the value. For example:

mem_used()
28.7 MB

# Operation 1 (I want to convert to GB)
mem_used()/1024
28.1 kB

# Operation 2 (I want to convert to kB)
mem_used()*1024
29.4 GB

But the operations above should have rather yielded for Operation # 1: '0.02803 GB' and for Operation # 2: '29388.8 kB'.

sessionInfo()
R version 3.2.1 (2015-06-18)
Platform: x86_64-apple-darwin13.4.0 (64-bit)
Running under: OS X 10.9.5 (Mavericks)

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] pryr_0.1.2

loaded via a namespace (and not attached):
[1] magrittr_1.5     tools_3.2.1      Rcpp_0.12.0      stringi_0.5-5    codetools_0.2-11 stringr_1.0.0