Reprojection problem

So here is my situation

I am doing a MLC and a reprojection step is required for the process to work.

When reprojecting, if I use the “Use CRS of” and I choose the product [1] or [2] as reference, the scene width and height will not match the ones of product [3], which is a subset. Although the reprojection works when using this option, it creates a product with a bigger extent and in a further step, when exporting to GeoTIFF, it will have almost 1 GB size.

The solution would be then to reproject and create a product with the appropriate extent. This could be done using the “Custom CRS” or “Prefered CRS” options of the Reprojection tool. However, when trying I face the problem described before.

I know a solution could be just to subset again the product with the big extent, but I wondering why the other options are not working.

Hope its clear
M