I agree that your child is likely just fine with this amount and will thin out when she becomes more active.
How about asking the babysitter what's behind her method? Has her experience taught her to do this in all cases? Does your child seem to get hungry soon after meals? Is she opposed to the idea of snacks and just opts to do all feeding at the three meal times? Regarding feeding at play time, maybe it's no big deal to her and she's discovered that everyone is happier when baby can eat while she plays. If this is something that you are particular about (Be clear in your mind about why.), then let her know your preference. Be prepared to give in on some things, though. Frame your questions as picking her brain for her expertise on how to achieve your goals.
Don't expect her to know your vision if you haven't shared it with her. Some things just aren't on some people's radars. If you want her to tend to your child properly, you'll have to let her bring some of her ideas to the table. Unless they are true no-nos for you, she'll need to figure some things out by trial and error just like you do. That will help her to get to know your child intimately.
I can't tell how often you use this babysitter, but I would have regular chats with my nanny. Not only did I she tell me what she was doing, but I explained to her my ultimate goals, and we discussed the best--and sometimes our differing--methods for reaching those goals.