When my LO starts bobbing on and off, and even fussing, I switch sides and she usually settles down for a long feed on that "other" side. Don't know exactly why, but I figure it might be because the breast she is on is getting empty.
And, yes, you will find that the older they get, the more distracted they get when BFing.

One trick I use is to sit beside a window where she can look out WHILE BFing rather than wondering what she is missing behind her. Or, as suggested, I go into the nursery where there are less distractions and I can pull the curtain.
If I just can't get her to settle down and stop bobbing, I don't worry about it as I know that she'll BF for longer the next time if she's really hungry. Let's put it this way - she lets me know if she's hungry and, like her mummy, she doesn't miss many meals.
Lastly, Lindsay has never grown proportionately. Just like adults, babies come in all shapes and sizes. My LO is in the 50th percentile for length and the 5th to 15th for weight, depending on the month. And my knickers aren't in a twist over it because she's healthy, active, well-tempered, sleeps well and has lots of full diapers.
HTH.