A well fitted bra can definitely help the distribution of weight with large boobs. I am a 36hh, All breast flesh needs to be pulled forward and contained within the cup. No spillage under the arm or out of the cup. The band needs to fit well around the back and never ride up as it's the band that takes the weight. To be properly fitted is a blessing and not all fitters are good, especially M & S who always try to fit me in a 38 or 40 band.
My properly fitted 36 band always feels tightish for the first couple of wearings...on the loosest hooks. After a couple of weeks I'm using the middle or tightest hooks and the well fitted bra is really doing its job.
If straps slip, it usually means the band size is too big.
I have always hankered for a breast reduction by means of a magic wand. Not so much...now that I have at last found what works for me.