Why analysts are already worried about the iPhone 6S even though Apple hasn't announced it yet

Apple hasn't even unveiled its next iPhone yet, but analysts are already expressing concern about whether or not it will be successful.

In a new note, Macquarie Research writes that it doesn't believe the next iPhone will drive as much growth as the iPhone 6.

The reason is simple - there's strong demand for large-screened phones, which is why the iPhone 6 was so massively popular.

The iPhone 6S isn't expected to be any bigger than its predecessor in terms of physical size, so analysts are weary about whether or not it will do well.

Here's what Macquarie writes (emphasis is our own):

However, looking ahead, we think that the comp issue becomes a much more significant issue in the September and December quarters. The fact is that there was significant build up demand for a larger screen heading into the iPhone 6 launch. While there are certainly many other factors impacting AAPL, this simple fact can't be overlooked. Demand for a larger screen drove much of the incredible growth in iPhone units over the past year. Since the new phone will not be a size upgrade, we are concerned that the growth simply won't be there. We are well aware that this is incredibly simplified, but because it is simple doesn't mean it won't impact the growth and stock.

The "comp issue" Macquarie is referring to boils down to the idea that since the iPhone is already doing so well, it will be hard for Apple to show growth there in the future. Macquarie isn't the only firm to express this concern - in a note from Monday, Piper Jaffray also noted the comps issue will be more prominent toward the end of this year.

Piper writes:

The well-known negative is that difficult comps starting in the Dec-15 quarter will soften investor optimism around iPhone growth.

Piper Jaffray also writes that iPhone growth is expected to go flat in CY16.

Cowen and Co analyst Timothy Arcuri didn't express concern about the iPhone 6S specifically, but he did explain that the iPhone 6 is different than other iPhones Apple has released because there was such high demand for a larger iPhone.

As a result, it will likely continue to sell well even after the new model is released, since Apple is likely to reduce its price. This relates to the comps issue mentioned by Macquarie Research and Piper Jaffray.

This is why Apple CEO Tim Cook talked about "switchers" so much during Apple's last earnings call - i.e. people who switch from Android to the iPhone. It's another metric Apple can use to show how the iPhone is growing.

Apple reports earnings later today, and we're expecting to hear about more stellar iPhone sales.