Two Layer File Save Enormous

When you changed it to 200 ppi, you did not resample the image - it just swapped size for resolution which is why you had a 1/2 size (W&H) with 200 ppi. You never actually changed the overall pixel dimensions of the image.  (121 x 99 x 100 ppi is the same as 65 x 49 x 200 ppi). If you want to reduce the size, turn on Resampling in Image Size then change your specs.  

Recreating your specs with W, H and resolution I ended up with a similar file size of almost 5 GB. That is what I would consider normal for something this large.

So, to add to this fun mystery, I opened up one of the other files I had that is using this same filter and instead just dragged this photo directly onto it and literally just stretched it bigger. It resulted in a BETTER looking image, I was able to blur/noise it much less to make it look acceptable, and the whole psd file is 1.2g (similar to the others). I don't understand why it would look SO much better doing it this way. Initially I was in a little bit of a panic because the original file wasn't looking as good as we really need it, but with time constraints, it was looking like we wouldn't have a choice.

Also, what’s the bit depth and have you done any transforms that add pixels outside of the canvas?

I can get a similar file size (3GB) on a 2 layer file by working in 16-bit and scaling up one of the layers:

The same document trimmed at 8-bit (342MB):

The following code tests for the presence of photoshop:DocumentAncenstor metadata, however, it takes a long time to respond if there is a lot of ancestor code. I'm sure that there is a faster way to test for the presence of this metadata using a script:

if (documents.length) {
   if (ExternalObject.AdobeXMPScript == undefined) ExternalObject.AdobeXMPScript = new ExternalObject('lib:AdobeXMPScript');
   var xmp = new XMPMeta(activeDocument.xmpMetadata.rawData);
   var metaBloat = getArrayItems(XMPConst.NS_PHOTOSHOP, 'DocumentAncestors');
   for (var a in metaBloat) {
      if (metaBloat) {
         alert('NOTE: There is DocumentAncestors metadata in this file!');
         break;
      }
   }
}

function getArrayItems(ns, prop) {
   var arrItem = [];
   var items = xmp.countArrayItems(ns, prop);
   for (var i = 1; i <= items; i++) {
      arrItem.push(xmp.getArrayItem(ns, prop, i));
   }
   return arrItem;
}

I had to take down the code samples as Blogger was mistakenly flagging my site as potentially malicious.

https://prepression.blogspot.com/2017/06/metadata-bloat-photoshopdocumentancestors.html

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Please show a screen shot from the Image > Image Size dialog. Maybe there's a detail you are missing.

Do you have any old backup of the file.  If you only have two layers and the document can only be saved as a PSB file that is over 4GB it sound like the document is corrupt or has Gigs of ancestor history that keeps growing.  If you have no backup you can find a script on this site that will strip ancestor history.  Use Google search this site search is useless.  Photoshop script strip ancestor history