Calories are the most important factor in muscle growth + maintenance
You need to get enough calories: I don't care what breatharian inclined fruitarians might say - wild Chimps get around 3000 kc per day, a good amount of which comes from nuts. With just fruits, say your requirement is a full 3000, you're looking at 30 x 7 = 210 oranges per week. So realistically you need something dense. I use coconut oil rather than nuts and avoid indigestion.
Also Chimps natural staple fruits are sweet varieties like wild figs, so I try to balance the oranges and lemons with blackstrap molasses.
You might want to search Andreas' forum for the keyword muscle. He explains how our bodies make protein mostly from the nitrogen in air. There are quite a lot of fruitarian and potato starch builders that are huge, Andreas Cahling for example. The potato starch guys have the idea that they have to eat carbs constantly to bulk, then trim; Andreas says that olympic gymnasts use coconut oil for both its fat burning and muscle building properties. I go with Andreas + Fruit - as a vata body type I find it difficult to bulk up, so this has really let me get ripped.
Also if your muscle is pumped, then it's filled with acid waste from too much exercise and cooked food. It *should* shrink during any type of detox. If you want muscle that doesn't do that, buy Pavel Tatsouline's Power to the People from dragondoor.com