It's generally not too difficult for a guy to gain speed by losing weight, but not usually by adding it. Maybin is still in his young 20s so it's maybe easier for him, but his size is that of a linebacker, but he's running like a wide reciever, albeit a slow one and this improvement is only in a 2 month time. You add an interesting point. He didn't gain weight for 3 years, now all of a sudden he gains 25 lbs (which is a lot) in only 2 months.
And dropping your 40 time .1 seconds isn't so easy. Obviously it's a big enough deal that it matters to the pros and for most people if you strap a 25 lb weight to their back they don't go faster.
I'm not saying it's impossible, but it strikes me as very difficult, plus I think you partly answered your own question. ...His career is at stake. He was smaller and slower and he stood to earn millions by quickly changing that. What was the downside? He wouldn't be the first guy who got caught using and was still drafted in the 1rst round.