PNG files are saving larger than they should be on macOS through Photoshop 2021

Hi Stephen, I wrote a script that opens an InDesign document’s image links via the Bridgetalk class and runs your metadata removal function.  It runs without error but I don’t have any bloated examples to test. I think the bloat can get transferred into the ID document, so I’m guessing you would have to save the ID doc as IDML after running the script to fully clear the metadata.

/*
* Clear ancestor metadata from document images
* Version 1.0
* Rob Day 2020-2021
*/

#target indesign
#targetengine "session" 

var doc = app.documents.item(0);
var lnks = removeDuplicates(doc.links.everyItem().name)

var lPaths = [];

for (var i = 0; i < lnks.length; i++){
    var l = doc.links.itemByName(lnks[i])
    if (l.parent.constructor.name == "Image" && l.status != 1819109747) {
        lPaths.push(l.filePath)
    } 
};   

//Send the array of file paths to Photoshop
runPhotoshop(lPaths);


/**
* Update the ID links after Photoshop saves 
* @Return void 
* 
*/
function upDate(){
    var lks = doc.links;
    
    for (var i = 0; i < lks.length; i++){
        //$.writeln(lks[i].name + " Uppdate")
        while (lks[i].status != LinkStatus.NORMAL) {
            lks[i].update();
        }
    };   
}


/**
* Opens Photoshop via Bridgetalk and run psScript() function
* @9397041 the array of file paths to open 
* @Return value void
* 
*/
function runPhotoshop(a){
    var bt = new BridgeTalk();  
    bt.target = "photoshop";  
    bt.body = "var a = "+a.toSource()+";"  
    bt.body += psScript.toString() + "psScript();";

    bt.onResult = function(resObj) {
        upDate()
    }  

    bt.onError = function( inBT ) { alert(inBT.body); };  
    bt.send(8);  
      
    function psScript() {  
        for (var i = 0; i < a.length; i++){
        var d = open(File(a[i]));
        //https://community.adobe.com/t5/indesign/file-size-is-too-big/td-p/9370587/page/2
        if (ExternalObject.AdobeXMPScript == undefined) ExternalObject.AdobeXMPScript = new ExternalObject("lib:AdobeXMPScript");   
            var xmp = new XMPMeta( activeDocument.xmpMetadata.rawData);   
            xmp.deleteProperty(XMPConst.NS_PHOTOSHOP, "DocumentAncestors");   
            d.xmpMetadata.rawData = xmp.serialize(); 
            d.save();
            //$.writeln(d.name + " Saved")
            d.close();
        }; 
    }  
}


/**
* Removes duplicates from the array
* @9397041 an array to edit 
* @Return an array with no duplicates 
* https://community.adobe.com/t5/indesign/js-how-to-remove-duplicate-items-from-an-array/td-p/3044978
*/

function removeDuplicates(a){
    var ra = new Array();
    var o = new Object()
    for( i=0 ; i<a.length ; ++i ){
        o[a[i]] = null;
    }
    for( a[i] in o ){
        ra.push(a[i]);
    }
    return ra;
}

Thanks for the reply. I'm on the latest version of Big Sur running latest photoshop through Rosetta on an M1 MBP. However, I have also previously noticed this recently on my iMac running the final update to Catalina. I'm saving the png through 'Save As' as I usually do and it's not a problem I can consistently reproduce.