Piezo Pick-up & String Tension

[tidalstream]

Good evening, I bought a Yamaha Silent Guitar, it's fitted with a piezo pick-up and steel string 12s. It had nice sound and an even volume across the strings. After one week I changed the strings to 10s and the volume is no longer balanced across the strings; the Low E String is louder than the rest. Is this normal? Do I need to go back to 12s to get a balanced volume?