There are at least 3 different explanations: The most plausible seems to be that a cake was 7 'liang' (16 liang in a jin, or a Catty, then equivalent to 1.3 pounds or 600g) and 7 cakes in a tong made 49 liang.
For sure, kilos and grams were not units of measurement in ancient China. Some people say that originally there were 8 bing in a tong and one was paid as a tax or levy. Certainly it was common for muleteers to pay (generally in goods) for safe passage through different areas.
Another explanation, that it was purely numerological, is possible; 7 is certainly a good number in Chinese culture.
I have yet to see a good explanation for why there were 6 or 12 tong in a set or jian, other than the previously mentioned 'how much a mule could carry' reason - it was thought that a mule or pony could not carry more than 60 catties - but the methods of pressing and packing tea have also varied over time.