I like Antman. Of course, it's a ludicrous premise and it's played that way by the leads. He doesn't have to stay the same mass, he could have a Higg's proportionality engine. I just invented that but I think it would work. It's really the only way to operate in the subatomic quantum realm (which is really just an excuse for some freaky CGI). As I've said before, all superheroes are ludicrous, it's only when they take it seriously (hear me DC, hear me) that it gets tedious or they have powers so profound there has to be, without fail, some kind of plot kryptonite. It's the Airwolf* Paradox (trust me, I just invented this too) – they invented a helicopter so awesome that the bad guys never had a chance, so it had to break down every week, it really less Airwolf and more Ernest Borgnine's Guide to Helicopter Repair. And frankly, who didn't want to spend Saturday lunchtime watching Ernest Borgnine fix a helicopter. Admittedly, I wanted to see Airwolf vs Blue Thunder. Still do.
SF that gets too hung up on (current) science gets dull quickly unless it's part of the plot and even then I don't mind liberties. I'm re-reading Alistair Reynold's Revelation Space series at the moment (real books, honestly I have Kindle-atrophied arms) which bounds the universe in relativistic travel, but even then the several kilometre long Ultra spaceships manage 99% of the speed light. I think that would take a fair bit of energy (but the get-out clause is the secret technology in the Conjoiner engines). The trick is to make even the implausible sound plausible. In my one-day-to-published (still working on that) SF opus I made some kludge involve complex maths and mutable reference frames. Utter scientific bilge of course, but if you're waiting for a bus to the other side of the galaxy, you're not going to get there any time soon.
*children, to Google you go.