Because you obtain your size in pixels and use it to translate to sp. First you need to convert size back to sp:
float scaledDensity = context.getResources().getDisplayMetrics().scaledDensity;
float sizeNew = text.getTextSize() / scaledDensity; // obtain current size in sp
sizeNew += 1f; // add 1 sp
text.setTextSize(TypedValue.COMPLEX_UNIT_SP, sizeNew); // set new size in sp