In case a Twitter post contains a HTTPS URL its remaining body will be limited to 117 characters to compensate for the extra “s”.
A tweet will therefore be shortened with 2 characters. While normal tweets can still be up to 140 characters long, tweets that contained URLs had a limit of 120 characters because of the t.co link shortner.
Starting with February 20, 2013 the t.co link shortner will generate links that are from 21 to 23 characters long. Until then t.co will generate links as before that use 20 to 22 characters. This change was expected as the link shortner gets more and more links added in its database and as time passes its generated URLs will be longer in size. Twitter might even decide to change its default URL shortner t.co or the way it works at a certain point in time.