If you go to the monitor section of the Screen Calculator you'll see that most monitors have around 90 ppi, a Full HD 1920 x 1080 24 inch - which is todays most commonly used external viewing device - has 92 ppi.
Your lower two devices have around 150 and the top one 224 ppi.
In order to make fonts and graphic elements visible, the manufacturers usually scale up the dpi by 120 or 130% in these small devices.
I have had an 8 inch Onda and currently use a 10.1 inch Pipo Max M9.
I always wish the screen would be bigger to reduce eye strain and therefore I am now eyeing an 11.6 inch Onda Vii6W dual boot (Android/Win8.1) tablet
(The link above is for a reputable dealer)
But really I would wish for a 13 inch tablet
. I feel no pressure and can wait for a few months. These new dual boot puppies are quite hot right now and will be a big game changer, I think.
I think Windows will gain ground in the tablet and phone market big times. Maybe we'll get a deja vu of the netbook development: The first ones came out with a specially adapted Linux distro, then Windows seriously started entering the game and within a short time you could barely even find the Linux ones. We'll see...
People report good things about the Win 8.1 on tablets though, and I think Win 10 will clinch the deal and Android's dominance will abate a bit. MS gives out the Win 8.1 Bing for tablets version almost free, so there is no more cost advantage for Android, but you'll get a task centered highly usable Win operating system with nearly seamless usability from desktop PC to notebook to tablet to phone.
I think it may work out for MS that way ...
(and I am writing this even though I am a big fan of Android)
Now the small investment with which one can obtain Android apps could work against it: My 100+ Dollar investment in apps versus the 10,000+ $ investment in software in the last thirty years won't be too much of an incentive to stay with Android (but it would be a strong discouragement to change to anything Apple).