You mean lossless as in "uncompressed" AVI?
Yes it's big, but you can always install a "real" lossless codec. The size was MUCH smaller.
I use "Ut video codec", not open source, but free.
Also, mainly because i can use it outside Adobe app, unlike QT codec.
If you didn't want to install/trust 3rd party codec like Ut, you can use Quicktime Animation or Quicktime PNG.
I've been using Ut for years. Also Ut can also be used on Mac, so you can exchange files with ease.
If you decided to use Ut, and want to encode higher than 8 bit per channel, make sure you open the "Readme For Adobe User" files by the end of installation, this is to add Ut to codec list that support 8bit+ encoding.
Or you can read here :
http://umezawa.dyndns.info/archive/utvideo/utvideo-16.1.0-readme-adobe.en.h...
If not, you can still use it in normal 8 bit.
Here's the link, download the latest version:
At the time of writing, it's v 16.1.0
http://umezawa.dyndns.info/archive/utvideo/
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Sorry for the english, not native speaker.