Is it common for a front tooth crown w/ a post to become loose & be replaced a year later with a larger, porcelain post?(photo)

The real question is why is the crown coming loose.  Is it a bite issue where too much force is being placed on the crown?   If it is a bite issue a bigger stronger post increases the risk of root fracture because a larger post demands more removal of tooth structure.  After the original fracture there may not enough tooth remaining to support the crown.  

You have not made it clear if the new post was placed , or only suggested when the new dentist sent you home with the crown still loose.  It does appear  from what you and the new dentist said that the original post may not be long and retentive enogh.  Without and xray or clinical exam, though, I cannot be certain.  It May also be possible to still use the present crown and retro-fit a new longer aesthetic post (I do not believe your dentist meant a porcelain post--just a non-metallic one that does not darken the porcelain crown.)  If needed send xrays with your next question.

Any time a post is done, it means that there is little tooth structure remaining so it is a compromise situation. The tooth really needs to be evaluated to see if it is hitting too hard when you bite which can knock it loose or if there is more going on such as a cracked root or decayed/fractured tooth around the post which is causing it to come loose. If that is the case, the tooth may not be saveable and it woyuld need to be removed and ideally, an implant done.See your dentist soon and find out what the cause of the looseness is.

It does sound like the post has become loose.  It is very important that you have this redone soon as you can get decay in the tooth that can quickly destroy what is left.  Find a good esthetic dentist who can determine if the tooth is savable and if so, what it will take to properly restore the tooth.  The sooner you have this taken care of better.  Good luck!