On your 1.5Mbps link, you could theoretically download 500GB per month. A huge amount, but I believe it was often genuinely allowed, because their uplinks could cope with it. Unlimited could genuinely be unlimited.

But now you might get things like “unlimited” 1Gbps… which reverts to 10Mbps (1% speed) or worse after 3.6TB (eight hours). And so your new theoretical maximum is about 6.8TB per month rather than 330TB.