In the data.lua file in the top directory of the zip file you'll find a loong section starting like this:
data.raw["recipe"]["copper-plate"].icon_size = 32
data.raw["recipe"]["iron-plate"].icon_size = 32
data.raw["recipe"]["stone-brick"].icon_size = 32
data.raw["recipe"]["gun-turret"].icon_size = 32
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edit every occurance of "32" to "64". This will tell the game to use the full icon images of the default game.