Many tiny documents in CosmosDB

What are the drawbacks of having many tiny documents vs the one aggregated document?

I would like to say that i suggest you storing each piece of data,instead of one aggregated document.

Reason1:As you mentioned in your question,if you want to add the element into the document,you need to read the document from CosmosDB, then replace the document because the partial update is not supported by cosmos db so far.(Please refer to this feedback and follow it if you need:https://feedback.azure.com/forums/263030-azure-cosmos-db/suggestions/6693091-be-able-to-do-partial-updates-on-document) That's a huge and tedious work.

Reason2:If you store pieces of data,you can query them flat. (select * from c)

For one single array document,you need to use join to access the nested properties.(select a.array from c join array in c.array)

Reason3:If you store pieces of data,you could manage them into different partitions.Even though you don't need it now,why not keep the feature for the future.

Reason4:As to cost,it all depends the RUs and storage and requests to cosmos db will consume RUs. If you store pieces of data,you just need to access the specific document as you want which is more economical i think.