Why would my new 2015 Macbook pro make more fan noise than my 2013 model? I've had a 2013 13" MacBook pro with a small SSD, 8GBs of ram and a i5 since I bought it new. I was always impressed by how I rarely heard the fan turn on at all during normal usage and when I did it was still very quiet. 

I just bought a used 2015 model 13" MacBook pro with a 2TB SSD, 16GB of ram and an i7. I haven't used it extensively yet, but I've already noticed the fan running much more frequently and loudly. I was just installing Xcode and it sounded like it was on full blast for the entire process. 

Is this to be expected because of the i7? Or due to some other difference in internals? Or is this indicative of some issue?

The computer is used but only has 260 battery cycles.
There's nothing about i7's in particular the determine that they should always lead to more fan noise than an i5. In many cases, i5's and i7's have similar thermal design power points.

Note that you're comparing an 2013 MacBook Pro with an i5 with a 2015 MacBook Pro with an i7. They're different in many ways. Therefore you cannot really say anything meaningful about whether it is due to an i7 or some particular other thing.

However, if you're annoyed by the noise, I would suggest cleaning the fans. Your laptop has potentially seen 4 years of use in an environment you do not know. The fans could be very dirty and that could in itself lead to a lot of extra noise. It might also be possible to reduce noise by reapplying cooling paste to the CPU.