Because a copy of your documents is always created on your hard disk, myCloud Desktop always occupies the same amount of memory as is used on myCloud.ch.
If there is not enough space on your hard disk to store all files from myCloud.ch, there is no need to delete files from myCloud.ch. Using selective synchronisation, you can exclude certain folders from synchronisation. Alternatively, you can change the storage location of your myCloud folder.
Nonetheless, if there is still no disk space available on your computer, you can always upload your files using the Web interface at www.mycloud.ch.