Calculating file size

File size is Mostly Pixel Data plus a little bit of meta data. So if you want an Uncompressed Tiff you cans calculate the Pixels data  and guess at the size of meta data . A RGB pixels requires 8bits for Red, 8bits for Green, and  8bits for Blue.  5000px x 3750px x 3 = 56,250,000 Bytes Dec.   Using 1024 as 1K computed  1024 for M  the works our the 53MB  for pixel data. So for a Flat Tiff the file size should be 53 Computer MB  Plus meta data size.. If the File is 74MB the file is either a layered document or there is a lot of Ancester metada which you can strip.

function deleteDocumentAncestorsMetadata() {  
    whatApp = String(app.name);//String version of the app name  
    if(whatApp.search("Photoshop") > 0)  { //Check for photoshop specifically, or this will cause errors  
        //Function Scrubs Document Ancestors from Files  
        if(!documents.length) {  
        alert("There are no open documents. Please open a file to run this script.")  
        return;   
        }  
        if (ExternalObject.AdobeXMPScript == undefined) ExternalObject.AdobeXMPScript = new ExternalObject("lib:AdobeXMPScript");   
        var xmp = new XMPMeta( activeDocument.xmpMetadata.rawData);   
        // Begone foul Document Ancestors!  
            xmp.deleteProperty(XMPConst.NS_PHOTOSHOP, "DocumentAncestors");  
            app.activeDocument.xmpMetadata.rawData = xmp.serialize();  
         }  
}  
//Now run the function to remove the document ancestors  
deleteDocumentAncestorsMetadata();