data.frame() size

Hi,

In the example below why is d 10 times bigger than m, according to
object.size ? It also takes around 10 times as long to create, which fits
with object.size() being truthful.  gcinfo(TRUE) also indicates a great deal
more garbage collector activity caused by data.frame() than matrix().

$ R --vanilla
....

nr = 1000000
system.time(m<<-matrix(integer(1), nrow=nr, ncol=2))

[1] 0.22 0.01 0.23 0.00 0.00

system.time(d<<-data.frame(a=integer(nr), b=integer(nr)))

[1] 2.81 0.20 3.01 0.00 0.00			# 10 times longer

dim(m)

[1] 1000000       2

dim(d)

[1] 1000000       2				# same dimensions

storage.mode(m)

[1] "integer"

sapply(d, storage.mode)

        a         b 
"integer" "integer" 				# same storage.mode

object.size(m)/1024^2

[1] 7.629616

object.size(d)/1024^2

[1] 76.29482					# but 10 times bigger

sum(sapply(d, object.size))/1024^2

[1] 7.629501					# or is it ?    If its not
really 10 times bigger, why 10 times longer above ?

r <- as.character(1:1e6)
object.size(r)

object.size(r)/1024^2