You guys are looking at the wrong problems though lol. Him being large and heavy isn't necessarily a flaw. You can't make Bowser small or light. He won't be Bowser that way. That's just how Bowser is; you can't focus on a problem that's literally not in your control at all lol. You are playing big, bad, Bowser. You have to get used to it. That's not something I would consider a problem that's been around for years, because if Bowser wasn't large or heavy, we probably wouldn't be playing the same character.
His main issues are that he cannot land without taking some kind of damage. If he had the airspeed he had in Brawl or better (the cast was smaller, so there was less competition), he would be able to escape to the edge fairly quickly and not have to be stuck in bad situations for long periods at a time. However, he doesn't have that. Even in Brawl with his pretty good air mobility and movement with Klawhopping, AND with the airdodge mechanics, he still had landing problems to a certain degree. Klaw is still a command grab that requires you to commit to a direction. That can get beat out fairly easily. And, ironically enough...as disjointed as Klaw was, it lost to more disjointed moves. Landing in Brawl goes deeper than that, and I could write paragraphs on how to land as Bowser in that game...he had options, but that's not important now.
He also has a blind spot below him. No aerial moves can help him get rid of that issue, because they all have some kind of risk behind them. The blind spot is below him, but the best spot to stay at to actively punish his landings is probably the diagonal angles from his position. You won't have to deal with DAir or Bowser Bomb, and you are always a threat to him. (And Klaw is pretty finicky now, so odds are if you're not playing me, you won't get grabbed :3) This is an issue that is mitigated by the "escape to the edge" solution. But again, because Bowser's airspeed isn't all that, it's kind of hard to get out of dodge from center stage unless you double jump, or if you hit the ground running. These are just things to keep in mind.
I have to add, though, that landing isn't necessarily impossible. Those highlight his main issues on paper, but players aren't perfect, and punishes get flubbed all the time. That's not to say that you should count on your opponent to make a mistake, but rather, count on yourself to be on point and be aware. If you do see an opening or an opportunity to get out of a situation, do it.
Being heavy in this game as Bowser honestly isn't bad at all. We get our early kills with rage, and good reads are rewarded more heavily when he has taken damage. He's like a fat Lucario...kind of. The only MUs where you really can't afford to take too much damage are those "on the clock" kind of MUs where you get combo'd and die to kill throws or throw setups. So like, Luigi and Ness, for example. I swear I mentioned this before.
As much as I would like to believe, Bowser doesn't currently belong in a "Top Tier" discussion. He's not that good, and he's not playing the type of neutral that the top 10/15 play (you would really have to pay attention to matches and character attributes to notice this. Top 15 is kind of stretching it, but I do believe the top 5-10 in this game are playing a different game) He's just okay. And we should be okay with him being okay now, because he was garbage for so long lol
I agree wholeheartedly with the sentiment that someone who puts time into Bowser will be able to place consistently and do fairly well, especially in Smash 4's current environment. It's not that hard to do well right now. I would definitely recommend to any (Bowser) player to get into Smash and support your scene now, while everyone and everything is still young. You will improve at your own pace and grow with everyone else, and the game will grow with you as well. The ability to place without really breaking a sweat will obviously change in the upcoming years, but you just need to remain calm, play solid, and just pay attention. I cannot stress this enough--good situational awareness will put you miles ahead of the average player in this game with any character. But Bowser rewards you heavily for solid play and solid reads under any conditions, and especially tense ones.