Use search engines with specific terms to find legitimate (or abandonware) sources. Do not click pop-ups.
Because you are focused on small files, you likely have a low-end device. Use these PPSSPP settings for heavy games like Peace Walker or Ghost of Sparta:
Lumines II (Size: ~250MB) A puzzle game synced to electronic music. Block dropping has never been so hypnotic. The low size comes from repetitive textures, but the gameplay is infinite. ppsspp games under 500mb download
Jeanne d'Arc (Size: ~480MB) A hidden gem tactical RPG from Level-5. Think Final Fantasy Tactics but based on the Hundred Years' War with anime cutscenes. It’s a full 40-hour strategy epic that fits comfortably under the limit.
Size: ~450MB - 500MB Genre: Fighting
Tekken 6 on PSP is arguably one of the best ports ever made. It retains almost all the content of the console version, including theScenario Campaign, but compressed into a tiny file size. The character customization and huge roster make it a must-have.
Size: ~450MB Genre: Action / Stealth
This game bridges the gap between Assassin's Creed and Assassin's Creed II. It translates the parkour and stealth mechanics of the console versions surprisingly well to the handheld format. It runs smoothly on PPSSPP with excellent texture filtering.
Genre: JRPG / Social Sim
Don’t let the "portable" subtitle fool you. This is a 70-hour epic. You balance turn-based dungeon crawling with a high school dating sim set in a creepy, time-looping city. The file size is tiny because the visuals are stylized 2D sprites over 3D environments.