Google recently posted a design guide Android Design:

So where do you begin when design­ing for mul­ti­ple screens? One approach is to work in the base stan­dard (medium size, MDPI) and scale it up or down for the other buck­ets. Another approach is to start with the device with the largest screen size, and then scale down and fig­ure out the UI com­pro­mises you’ll need to make on smaller screens.

Directly from the horse’s mouth: On Android UI com­pro­mises must be made. I under­stand com­pro­mises need to be made to opti­mize UI to a cer­tain plat­form. The UI used on a con­ven­tional PC will not work very well on a smart­phone and vice versa. But UI com­pro­mises should not exist within the same plat­form. Here Google clearly admits UI com­pro­mises are nec­es­sary when devel­op­ing apps for Android. If Google wants to sup­port mul­ti­ple dis­play sizes it should have devel­oped a frame­work allow­ing for a con­sis­tent user expe­ri­ence across dif­fer­ent dis­plays. Obviously UX con­sis­tency was not Google’s top pri­or­ity. If it was there would have been and con­tinue to be tremen­dous effort toward mak­ing Android res­o­lu­tion inde­pen­dent by mov­ing to vec­tor text and graph­ics that do not depend on ppi.

There are four DPI lev­els (MDPI is baseline):

• LDPI: ~120DPI
• MDPI: ~160DPI
• HDPI: ~240DPI
• XHDPI: ~320DPI

What this means is that when res­o­lu­tion increases from ~120 ppi (I pre­fer ppi when dis­cussing pixel-based dig­i­tal dis­plays) to ~159 ppi fonts and icons will get smaller. This applies to all three lev­els of in-betweens. And dis­plays north of ~320 ppi will con­tinue to get smaller with no reprieve.

When the 960×640 3.5-inch Retina Display was intro­duced with the iPhone 4, the rest of the smart­phone indus­try had to do some­thing about it. At the time most of the com­pe­ti­tion was at 800×480 on dis­plays much larger. With the Retina Display Apple shifted the focus of atten­tion on a smart­phone dis­play to res­o­lu­tion, specif­i­cally to a res­o­lu­tion thresh­old of about 300 ppi on a smart­phone that’s used at a dis­tance of about 12 inches.

Here is a list of Google phones (date intro­duced, name, dis­play size, pixel for­mat, resolution):

• Sep. 2008: G1, 3.2 inch, 480×320, 180 ppi
• Jan. 2010: Nexus One, 3.7 inch, 800×480, 252 ppi
• Dec. 2010: Nexus S, 4.0 inch, 800×480, 233 ppi
• Nov. 2011: Galaxy Nexus, 4.65 inch, 1280×720, 316 ppi

If dis­plays size were kept at 3.2 inches from the G1, the cor­re­spond­ing res­o­lu­tions (ppi) would be:

• G1: 180.23
• Nexus One: 291.55
• Nexus S: 233.24
• Galaxy Nexus: 458.94

On the G1 the base­line MDPI (~160 ppi) would have fit quite well. With HDPI (~240 ppi) the Nexus One would have had small­ish UI ele­ments. The Nexus S would have been a very good fit for HDPI (~240 ppi). The UI ele­ments on the Galaxy Nexus on the hand, even with XHDPI, would have been microscopic.

Android OEMs and Google responded to the 3.5-inch 960×640 Retina dis­play by improv­ing the pixel for­mat to 1280×720. But because Android ren­ders text and graph­ics like desk­top OSes (e.g. Windows, OS X) increas­ing res­o­lu­tion above 320 ppi means smaller UI ele­ments. The dis­play had to grow in size to com­pen­sate for shrink­ing UI ele­ments. iOS ren­ders the Retina dis­play not by shrink­ing UI ele­ments by one fourth but by dou­bling clar­ity and sharp­ness. Unless Google adds an addi­tional “DPI level” beyond XHDPI, Android smart­phones that match or beat the iPhone 4/4S in res­o­lu­tion will always be big­ger, much bigger.

Update: Jeremy E. wrote in:

It is also very pos­si­ble for Android to sup­port the iPhone 4′s retina dis­play met­rics because it sup­ports 320ppi.

He has a point. UI ele­ments on a the­o­ret­i­cal 3.5-inch 960×640 Android smart­phone using XHDPI would only be slightly smaller com­pared to a 3.5-inch 960×640 iPhone. I guess what I was try­ing to answer was: Would a 1440×960 iPhone be a big­ger phone? My guess is no. Apple would improve clar­ity by 50% and not increase dis­play size. Another way of say­ing it is Apple would add a 3x base­line ppi so dis­plays need not get larger to com­pen­sate for shrink­ing UI ele­ments. (Just a guess at what Apple might do in that sit­u­a­tion. In all like­li­hood 1440×960 will prob­a­bly not exist on an iPhone, but we all know no one really knows what Apple might do.) Then why are 1280×720 Android smart­phones much big­ger? Because UI ele­ments would get too small if they were much smaller than 4.5 inches.

The bot­tom line is there is a dif­fer­ence between how Apple improves the visual expe­ri­ence on the iPhone and how Android smart­phone OEMs do it. Apple has improved dis­play qual­ity on the iPhone while keep­ing it the same size while Android smart­phone OEMs have com­peted with Apple by mov­ing to 1280×720 and much big­ger dis­plays. John Gruber offers a dif­fer­ent the­ory as to why Android smart­phones are big­ger than the iPhone.