How much mortar between bricks?
Typical mortar joints are 3/8", but there is no rule for it, that it has to be that size. They can be much wider than that. I have seen joints as wide as you have pictured, no big deal, it is a matter of aesthetics.
The mason could have looked to find the right size brick, but he may had to order them and possibly pay a premimum for them, especially if he is buying less than a cube(about 500 brick) much tougher when buying only a 100 or just a few dozen, so perhaps he went with what he could get. 

The brick he is using may be 7 5/8" long, which is a "modular". The original brick may be 8" long or "standard" brick, from what I found on the internet. There is no way for me to know, it is an educated guess.