Hello,
Welcome to community!
Thin- It will consume only that much space that it need initially and will grow based on the demand later. Ex you Provisioned a VM with 4 GB but it need initially only 2gb so it is going to consume only 2gb and can grow later till 4gb because that what u allocated.
Thick- Will consume the entire space from the datastore for example if you provisioned disk with 4 GB straight away gone from the datastore dedicated for it.
Kindly mark it as correct/helpful if this answer your query.
Rgds
Kanishk