[otnt]

Of course, I could delete all pieces with an unwanted cover in MobileSheets and upload them again after cutting of the cover of the sheet music, yet I do hope there would be a much quicker way.

Thanks in advance for your kind and helpful answer.

You already imported large file.
There's no other way but only to change the size of your primary source.

Google's desktop version of Chrome browser is good keeping the PDF size down during the conversion.
Print->Save as PDF->exclude art pages (keep original paper size)