I've ordered the ovo-lacto vegetarian meal a number of times, in both economy and business. Generally the portion sizes have been comparable to normal meals, but I've been disappointed in the flavor.

My guess is that for the sake of simplicity and cost, the food service companies often have meals that satisfy a number of special meal requests, as the ovo-lacto meals typically seem to meet the requirements of vegan, bland, low-fat, and low-sodium too. The typical meal is often something like steamed or grilled vegetables and rice pilaf (with little or no oil or salt), small salad with low-fat dressing, roll, margarine, and a bit of fruit for dessert. A pre-arrival snack may be a plain, whole-grain roll with some tomato and cucumber on it. I'm sure these meals are wonderfully healthy for me, but they don't have much flavor and don't leave me feeling very full.

Fortunately these days, US seems to be including a meatless meal more often on their standard menu (Virgin Atlantic has been doing this for some time). They specifically mention vegetarian meals for Envoy on their website, though I'm not sure if they're regularly on the economy menu. These "regular" meals tend to be more rich and flavorful, being something like vegetarian lasagna or cheese-stuffed shells (don't get too excited -- they're still inflight meals). Of course, depending on this option has some risk, as US can make changes to their menu offering without notice, and they may simply run out of meatless "normal" meals before getting to you. Still, more recently I've been willing to "take the risk", while having some back-up snacks of my own in reserve.