Besides the obvious difference of an additional 8 players in the 15s game, the duration of the game is a key difference.
The pitch is the same size in both games, and rules also match. The 15-player version is played over 80 minutes, split into two halves, whereas 7s is just seven minutes each half.
There’s no doubt that 15s is a high impact sport – the first tackle you see will convince you of that, but the pace is not the same as 7s. Think of it as the marathon/sprint analogy. 7s players have to cover the same ground as their 15s counterparts, but with fewer teammates to back them up and help out.
15s play, generally, one match a week because it takes longer to recover from that sort of intensity, but another big difference between the two versions of rugby is that 7s will play a tournament over two days at each competition.
Pace has the edge in 7s because, in the 15 player game, much more focus is given to the set piece. 7s simply couldn’t commit the whole team to a scrum, so their scrum only uses 3 players. It’s much more of a way simply to restart the game, rather than having the weight of tactical significance that it has in rugby. 15s players will spend hours fine-tuning and be honing their skills, often using a rugby drill video for each different skill, from experts like Sports Plan https://www.sportplan.net/drills/Rugby/.
The same applies to lineouts – who would receive the ball if everyone was committed to a lineout? So there are no driving mauls, and there is less support at the breakdown in 7s, simply because there are not enough players on the field to cover the pitch.
7s players have two options for moving the ball between players; they cover more ground themselves, or they make a long pass. Neither version has players of any less skill, they are just focused in different areas.
There are also implications for how the two different versions of the sport are coached, as outlined on the World Rugby website.
Rugby is all about tactics and strategy, whereas the shorter version of the game sees the focus on possession. The values of teamwork and respect are still held by both games.