Almost everyone is less efficient when climbing out of the saddle (OTS). That's because you recruit more upper-body muscles, which takes energy, which means you will experience higher HR for a similar power output, stressing the CV system. Hence getting out of breath. Very few can climb OTS for 25 minutes at a stretch unless they are taking it pretty slowly. Your standing power will match or exceed your seated power because you are using your bodyweight, but only for short periods.

Having said that, there are issues of technique. It's difficult to maintain a high cadence OTS without simply becoming exhausted, so change up one or two gears as you stand. Stand as straight up as possible, so that most of your weight is directly above the bottom bracket and as little as possible is on the bars. Get into the sort of rhythm you might use on a stepping machine in a gym, maybe 60rpm rather than the 70 or 80 you might use seated. Climb on the hoods, Keep your body near vertical and let the bike rock from side to side between your legs. Don't fight too hard to keep the front wheel straight, it's normal for it to weave slightly as the bike sways. As always, relax the upper body as much as you can.