Ich sollte weniger essen?
Shouldn't this mean (past tense) "I should have eaten less" not "I should eat less?"
One more thought on understanding this. You can say "Ich würde" (I would) or "Ich sollte" (I should). In German, you can also say "Ich soll", which can also be translated as I should, but in German "ich soll" is not subjunctive. It has a meaning more like "I must", "I ought".