Reserved memory of R is twice the size of an allocated array

I spotted the following behavior. Say I create the following multi-dimensional array:

spam = array(runif(96*48*60*360), dim = c(96,48,60,360))

It is quite predictable how much memory R should use for this, namely (96*48*60*360) * 4 bytes = 759.4 Mbyte. This is nicely confirmed using the lsos function (see this post):

> lsos()
         Type      Size PrettySize Rows Columns
spam    array 796262520   759.4 Mb   96      48
lsos function       776  776 bytes   NA      NA

R as a process however uses much more memory, roughly twice the size:

$ top | grep rsession
82:17628 hiemstra  20   0 1614m **1.5g** 8996 S  0.3 40.4   0:04.85 rsession  

Why does R do this? I assume the extra reserved memory is allocated to make it more quickly accessible to R? Any thought's?