Generally, in very large aircraft, engine power-up comes in increments, with a "stability-check" and then a bit more and then to the desired N1 limit (among other factors). That
might explain your "sudden increase in thrust". Quite soon after takeoff, sufficient power is obtained and minimum safe altitudes are reached and thrust will be cut back, giving engine-efficient climb thrust settings. Doesn't seem out of the ordinary to me, but again, I wasn't there!
Hope this helps,
SFly