To know how to respond, you first need to know why you want to respond. You stated that it starts to get a little irritating. Two points about that. First, is it irritating only you, or is it also irritating others when it happens? Second, what about that behavior is actually irritating you? The fewer people who are irritated by the behavior, the more important the second question becomes. Still, for a mostly polite response you need to know why it irritates you anyway, since that is what you will need to identify to them when you do address it.
For politeness sake, you have to address it in a private conversation. That implies that it will be after the fact rather than in the moment. Doing so allows you to; a) consider what, and how, you're going to say, and b) allows them to get the ego rush, if that is why they do it.
When you do have the conversation, you need to be sure that you are speaking for yourself, even if the other double-dozen listeners were irritated as well. Unless the group has elected you as their spokesperson for this situation, you only represent yourself. You also need to take ownership of your feelings and emotions in the situation, rather than blame the other person for "causing" them. That is, no other person can make you feel a certain way; rather, you choose, all be it not at a conscious level, to feel certain emotions in response to given stimuli. After owning your emotional responses, you are able to indicate the behaviors you responded to.
Properly modified for the situation, and for what your real emotional response was, you could begin something like this:
You know, Bob, I've got this problem I could use your help with. Sometimes lately I've been feeling like I've been discounted, or that my experiences are not worth sharing. I get to feeling this way when I hear about experiences other people have that make mine seem so much smaller, especially some of your recent adventures.
From there, you can see if they will move into a mental position that allows you to encourage them to modify their actions, or if it would be a waste of your time to try. If it's going to work, they'll meet you halfway, and the conversation should carry itself from there.
There are a few important points in there.
- You have a problem, not we, the group, others, etc., just you.
- You are asking for their help.
- You are expressing your feelings; the emotional context is important.
- You are not attacking their actions, not even identifying a specific instance.
- You have not blamed them for how you feel.
- You have not given them the need to defend themselves, or their actions.
There are also a few things you can do to improve the encounter, and its chance for success. As said earlier, make sure it's a private conversation, not a whispered chat in the corner of the room, but a separate space, another room, etc. Prefer sitting, but sitting or standing, make sure that you are not somehow "above" them. If you're taller by more than a couple inches, sit with them. However the two of you are positioned, be sure that their back is protected and that they have the best, and clearest, exit route. The exit route, the equal level and the privacy all help to keep them from feeling trapped or feeling the need to become defensive.
In the end, especially if it's only you that is becoming irritated with their behavior, there may not be a way to change if, and you will have to either find a way to not be irritated when it happens, or grin and bear it until you can leave the situation.