It is not possible to change the image resolution when using quick actions. The output layer gets reduced to about 44% of the original size for additional editing within Adobe Express but the starting image dimensions should be the same. You should be able to resize size the output layer closer back to the original size if you wanted. If you'd like the have the remove background work a differently please create a feature suggestion for our developers here for consideration: https://creativecloudexpress.uservoice.com/forums/329007-adobe-express-feature-suggestions
Thank you very much for your reply. It is very clear. It also happens to me that when I remove the background, the image remains with the size of the deleted background, which means an extra step when working with the downloaded image. Do you think this is normal? Thanks again for your response. Despite these details, adobe express is a great tool!