Steam compresses games as much as possible, so in the case of Helldivers 2, you had to download between ~30 and ~40 GB, which was then unpacked to 150 GB (according to SteamDB[0])
Some powerful retro handhelds support Linux loading, such as: Retroid Pocket 5, Mini, and Flip 2 on the five-year-old SD865, and more recently, Ayn Odin 2 (original, Mini, and Portal) on the three-year-old SD8 Gen 2 (which is one version lower than the SoC in Steam Frame (SD8 Gen 3)).
So if we get a native arm version of Steam and Proton ARM64EC, we will essentially already have mini Steam Deck(s), and since you want something similar to a PSP, you can check out the Ayn Odin 2 Mini, it's similar to the PS Vita, but I'm not sure if it's still available for sale, or you can order the Retroid Pocket 6 (available in a few months), which has the same chip, but a better screen and is also small in size.
Steam compresses games as much as possible, so in the case of Helldivers 2, you had to download between ~30 and ~40 GB, which was then unpacked to 150 GB (according to SteamDB[0])
[0] https://steamdb.info/app/553850/depots/