technically no you cant, because flexing is moving
but for full contraction of your pecs notice where your hands are
Flexing simply means a muscle contraction, you didn't say anything about a ''full contraction''. I think there's not need to over complicate things in bodybuilding, if a bodybuilder can bench press 200 lbs, as his max, and he eventually gains strength, and gets to bench 500 lbs, I guarantee you his chest will be much bigger, provided he is growing in proportion, meaning he doesn't let his delts and triceps grow out of proportion, there are bodybuilders who can bench a ton of weight but their pectoral muscles are not that impressive, why? look at their physique, they have huge delts and triceps. It's important to grow symmetrically