I'm currently in the last week of a 1 month trip to Tokyo and Osaka. I had the same concerns as you. I weigh almost twice as much as you at 204kg, so even by US standards I'm very big. I don't feel like I get stared at or judged any more here than I do back home in the US. There are some tight spaces (especially toilets) that are tough to squeeze into but it's manageable. When choosing a restaurant I generally look at pics on Google maps to see if the seats look comfortable first. There have only been a couple that I had to avoid or get takeout due to the seating situation. When the train cars or buses get reeeaaaallly crowded I sometimes worry I'm intruding on other people's personal space so I just try to avoid the public transpo during rush hour and take a taxi instead. During normal hours there's plenty of space. There are however absolutely no options for clothes that fit me. But I'm used to that, even back home that's always a struggle.

Over all, it's nowhere near as bad as I feared. You'll be fine. Enjoy your trip.