Photoshop script change textitem size (javascript)

When i reduce size, the text size will bigger.

app.preferences.rulerUnits = Units.PIXELS;  
app.preferences.typeUnits = TypeUnits.PIXELS;  

var doc = app.activeDocument;

var textLayer = doc.layers.getByName('obj-name');
var tI = textLayer.textItem;
var old_size = tI.size;
var size_value = old_size.value;
size_value -= 1;
tI.size = new UnitValue(size_value , 'px');

$.writeln(tI.size + " " + old_size + " " + size_value);

the result is:

134.722198486328 px 33.3333282470703 px 32.3333282470703

My ps version is 2020.