In Selecting the right bento box , I talked about how important it was to select the right size of box, especially if you are using bento lunches as a tool to lose weight. The other critical factor is to control the amount of rice you put into the box, if you are making Japanese style bentos.
Most regular Japanese bentos have quite a lot of rice in them - typically, at least 2 cups, compressed tightly to make room for the rest of the food (the okazu). 1 cup of medium grain brown rice (240ml, or 1 U.S. cup measure; around 7 oz or 190-200g in weight) is around 220 calories, so if you stuff in the rice following the norm, that’s quite a lot of calories. This is why I like to pre-portion the rice (by freezing  or not) and limit myself to 1 cup per bento. (For my bigger spouse, I put in 1 1/2 to 2 cups, depending on what else is in the bento.)
Another factor to consider is that since typical Japanese bentos have so much plain rice in them, the okazu (the other food) tends to be quite salty. If you have less rice, you will not want as much saltiness because you don’t need it to make the rice “go”. (There’s even a common phrase for this to indicate if something is tasty to have with rice - gohan ga susumu). I try to keep the salt content down in the bentos I post here. Another tactic I use is to add vegetables that are barely seasoned, such as the plain boiled green beans and carrots in Bento no. 1 .
Watch out for the saltiness in things like prepared pickles and furikake powder too.
[Edited to add:]
There is a small caloric difference between different types of rice. And as a commenter pointed out, I forgot to mention that by ‘cup’ I mean 1 U.S. cup, or 240ml. (For the sake of simplicity I just remember the numbers for brown rice, which is a bit more caloric than other types of rice. Overestimating a bit doesn’t hurt!)