Top picture seems to be big-big. Bottom picture appears to be small-small. Top picture looks like there is some way to go before the rear dérailleur is fully extended. Guess your chain is too long. Rumour has it you ignore the dérailleur, wrap the chain big-big and add two links in order to get the right length. I may be wrong about how many links you add... I may be thinking too simplistically but I would be surprised if any stretch you measure would result in the the rear derailleur being under extended to the extent that it currently appears to be.