Recordings -Shared window show up smaller

Welcome to the Zoom Community, @VirginieDutrey.

Do you know what size the recordings are?  Usually you can look at file properties, and for Full HD it should read 1920x1080.

Are you normally sharing full screen (as opposed to partial screen shares)?  Do you share from different monitors?

Recordings of screen shares are generally made at the same resolution as whatever is being shared.  So if you are sharing a portion of your screen, only that rectangle is being recording.  If you are perhaps recording from a presentation on old monitor that is 1280x720, that's the size the recording will be.  If there are multiple screen shares at different sizes, the FILE format will be the largest dimension of any screen share, and any shares smaller than that will be padded with black pixels and only the "native" screen size will be visible.