Spence (24-0, 21 ko's) is a 147 fighter, but could easily fight at 154. In fact, he'll probably end up as a middleweight at some point in his career. He's a southpaw, has a great temperament, power, terrific body punching and is fighting at home in Texas. Garcia (39-0, 31 ko's) is probably most comfortable at 135. He started off at 126lbs. Spence has a 4" reach advantage and 3.5" height advantage. Against Brook I noted how Spence, despite his lack of 12 round fight experience, got stronger as the fight progressed. He actually picked up the pace in round 8 and never relented until he got the stoppage in round 11. He was great at cutting off the ring as he stalked Brook. Yes, Brook had a fractured eye socket along the way, but Spence turned into an absolute animal and has that in his locker if he needs it. He's quite the athletic specimen. If Garcia somehow manages to control things in the early to middle rounds with his boxing, I could see Spence going through the gears and battering Garcia into submission late. Garcia lost his power when he moved up to 140lbs and Spence has a good chin and improving defence. It leads me to believe Garcia can't win by ko.
Garcia took two and a half years off from 2014 to 2016. I get the feeling he's trying to make up for lost time by challenging Spence. He's skipping a few steps and I believe he will pay the price as a result.
I think Spence v Algieri reveals much of what will happen against Garcia. Algieri showed a lot of feints, used his legs a lot, but couldn't hurt Spence. Spence used intelligent body punching and broke him down early. In fact he's the only man to ever stop Algieri. It was a fantastic performance. I remember seeing him land a beautiful body shot when Algeiri was trying to sneak off the ropes and away to the side. Body shots will play a big role in his fight with Garcia. Now Algieri doesn't hit as hard as Garcia will, but surely Brook does. Power never seems to bother Spence. If Garcia can't hurt Spence then he won't be able to keep him off.
Garcia v Lipinets told me everything I needed to know about Garcia. He doesn't have the mobility to evade the attacks of Spence and although he has some bounce, could almost be described as flat footed. He'll struggle to carry around the extra poundage for 12 tough rounds. I also noted that Lipinets caught Garcia at an unexpectedly high rate, winning 3/4 rounds along the way. That doesn't bode well.
In an interview with Lomachenko he didn't give Garcia much hope. He says the size difference will just be too much. Professional fighters know. When Rigondeaux moved up 2 weight classes to fight Lomachenko, he got wasted. It happens time after time. De La Hoya and Trinidad against Hopkins, Mosley against Wright, Brook against Golovkin, Khan against Alvarez. Sometimes fighters pull it off, but most of the time they don't.
On fight night I'd expect Spence to weigh, at a minimum, 165lbs. The IBF have a policy whereby neither fighter can re-hydrate more than 10lbs between the official weigh-in and the weigh-in the day of the fight. Sounds restrictive, but the second day weigh in takes place between 8am - 10 am. That would give Spence another twelve hours to re-hydrate and pile on the pounds. I don't see any benefit to Garcia putting on much more weight as he's going to have to be light on his feet and keep his speed. I'd guess he'll weigh around 150lbs.
I think the bottom line is that Spence isn't an ordinary welterweight. He's a super welterweight masquerading as a welterweight. He's also a killer in the division. He hasn't had to go the distance in the last five years. Garcia can pack on all the muscle he wants, but it's asking too much to jump up yet another division and expect his skills, athleticism and power to come with him. If he were fighting any other welterweight, with the exception of maybe Crawford, I would have given him a chance. He's not and I don't.