Event calendar's block changes size automatically

Sorry if this issue is similar to other, I didn´t found something like this.

When I access my site as an administrator, there is no problem with the event calendar block (as seen in c1.jpg).
But when I access the site with a common authenticated account this happends:

In the main page its all right (like the previous image) but when I enter some sections (like my account, messages, etc) or contents (a page content for example) the event calendar block changes its size (as seen in c2.jpg). From this point, the new size of the block remains (even in the main page) untill I log out with the current account.
When I enter the site again with this last account, this behaviour repeats.

Please help!!!