ost file and connected mode on exchange

hello

I explain my problem
normally, the ost file is created only when using cached mode on exchange server.
link:
http://office.microsoft.com/fr-fr/outlook-help/presentation-des-fichiers-de-donnees-outlook-pst-et-ost-HA010354876.aspx

My test is as follows:

I create a profile where I connect without caching on the exchange server, I do not actually have ost file created.
I decided to switch the profile into caching mode, the ost file is created (220 MB).

I finally decided to change the profile and returned to the first confiuration (whitout cache), I rename the ost file, change the configuration and restarts outlook. and then I realize that outlook recreates an ost file (140MB smaller).

my question is: why outlook recreates an ost file and why does it not have the same size?

thank you in advance for your answers