Dropbox and Google Drive keep a local copy unless told not to

Both services can be configured so the cloud costs almost nothing locally, and both ship defaults that put a full copy on your drive. On a 256 GB laptop with a 400 GB team folder, that difference is the whole machine.

Google Drive: mirroring against streaming

Streaming mounts the drive as a virtual volume, usually G:, and downloads on demand. Individual folders can still be marked Available offline where you need them, which is the combination worth aiming for.

# The on demand cache, which exists even in streaming mode
$p = "$env:LOCALAPPDATA\Google\DriveFS"
'{0:N1} GB' -f ((Get-ChildItem $p -Recurse -Force -File -EA SilentlyContinue |
  Measure-Object Length -Sum).Sum / 1GB)

The cache is capped in Drive settings under Local cached files directory and its size limit. Moving that directory to a second drive is the better move on a machine with a small system disk.

Dropbox: selective sync and online only

1. Selective sync decides which folders exist locally at all. A folder excluded here occupies nothing.
2. Make online only, on a folder or file, keeps it visible in Explorer with the content in the cloud.
3. Smart Sync defaults can be set per account so new folders arrive online only rather than downloading.
4. The cache lives in %LOCALAPPDATA%\Dropbox and holds recently changed files for three days before clearing itself.

$p = "$env:LOCALAPPDATA\Dropbox"
Get-ChildItem $p -Directory -Force -EA SilentlyContinue | ForEach-Object {
  $s = (Get-ChildItem $_.FullName -Recurse -Force -File -EA SilentlyContinue |
        Measure-Object Length -Sum).Sum
  [PSCustomObject]@{ MB = [math]::Round($s/1MB); Path = $_.FullName }
} | Sort-Object MB -Descending | Select-Object -First 10

The trap both share

Online only is not the same as absent. Opening a file downloads it, and it stays downloaded until something evicts it. A week of working through a large folder quietly converts most of it to local copies, and the drive fills again with nobody changing a setting.

This is exactly the behaviour described for OneDrive and Files On-Demand, and for iCloud for Windows. If two or three of these run on one machine, each keeps its own cache and its own copy, which is worth checking before blaming any single one.

A sensible arrangement on a small drive

• Streaming or online only by default, with offline marked on the two or three folders you genuinely need.
• The cache directory moved to a second drive if there is one, and capped if the client allows it.
• Shared team folders excluded entirely unless you work in them this month.
• A check after any week of heavy work, since usage converts files to local copies silently.