consider also the screen size when scaling by DPI

frontend/device/kobo/device.lua

@@ -52,7 +52,7 @@ local KoboPhoenix = Kobo:new{
     model = "Kobo_phoenix",
     hasFrontlight = yes,
     touch_phoenix_protocol = true,
-    display_dpi = 212.8,
+    display_dpi = 212,
     -- the bezel covers 12 pixels at the bottom:
     viewport = Geom:new{x=0, y=0, w=758, h=1012},
 }

frontend/device/screen.lua

@@ -169,8 +169,11 @@ function Screen:setDPI(dpi)
 end
 
 function Screen:scaleByDPI(px)
+    local dpi = self:getDPI()
+    -- larger screen needs larger scale
+    local size_scale = math.min(self:getWidth(), self:getHeight())/dpi/3.6
     -- scaled positive px should also be positive
-    return math.ceil(px * self:getDPI()/167)
+    return math.ceil(px * size_scale * dpi/167)
 end
 
 function Screen:getRotationMode()