Gmail tells you exactly what it is about to do, in a sentence almost nobody reads. You run a search, tick the box at the top of the list, and a small line appears:
Select all conversations that match this search.
Conversations. Not emails. That one word is the difference between a clean inbox and a hole where a project used to be.
A search matches a thread, not a message
In conversation view, Gmail's unit is the thread. A search hits a conversation if any single message inside it matches, and the whole thread comes back as one row.
Most of the time this is harmless, because the mail you want gone does not thread. A newsletter, a receipt, a shipping notification and a no-reply alert are almost always one message each, so one row really is one email. That is why category sweeps behave the way you expect:
category:promotions older_than:1y
from:no-reply older_than:6m
Size sweeps are the ones that bite:
larger:10M older_than:2y
has:attachment larger:5M
A 40 message thread with your team, where one person attached a 12 MB PDF somewhere in the middle, matches larger:10M. It appears as a single row. Select it, hit delete, and all 40 messages go to Trash, including the replies you wrote yourself, because your sent mail lives inside the thread too.
So the row count is not the message count, and the two can differ by an order of magnitude in either direction. You can delete 200 rows and remove 2,000 messages.
Turn conversation view off before a risky sweep
Gmail has a setting for exactly this, and it is reversible and non-destructive.
Settings, See all settings, General, Conversation view: off.
Nothing gets deleted and nothing gets permanently unmerged. The display changes to one row per message, which means the number in the results is the number you are actually selecting. Run the size sweeps with it off, look at what is really there, then turn it back on when you are done.
If you only want one message gone and the rest of the thread kept, Google's route is to open the conversation, find the message, click the down arrow next to Reply at the top right of that message, and delete just that one.
Threading is not what you think either
Subject line alone does not thread two messages. Since 2019 Gmail also wants the incoming message's References header to point at IDs of earlier messages in the thread. Two unrelated emails from the same sender with the same subject often stay separate, and a long thread can split if a client along the way mangles the headers.
The practical consequence is that your mental model of "this is all one conversation" can be wrong in both directions, which is another argument for looking at the flat view before a bulk action rather than after.
The rule that falls out of this
Sender and category based cleanup is low risk because that mail does not thread. Size based cleanup is high risk because big attachments cluster in exactly the long human threads you want to keep.
If you take one thing from this: do your storage recovery by sender first, and only go hunting for large attachments once the easy stuff is gone and you can afford to check each hit individually.
Full disclosure, I build a Chrome and Firefox extension in this space, Gmail One-Click Cleaner. It leans on the same logic: the preset sweeps are the sender and category ones, it shows a Mailbox Report of what it found before anything is deleted, and it keeps a recovery log so a bad sweep is one click to undo. There's a Firefox build as well. It runs locally and never sends your mail anywhere.
But the setting toggle above costs nothing and works whether you use any extension or not. Flip it before your next big delete.
