You define weights in haproxy to spread the load according to your needs.
From the official haproxy 1.4 documentation:
The "weight" parameter is used to adjust the server's weight relative to
other servers. All servers will receive a load proportional to their weight
relative to the sum of all weights, so the higher the weight, the higher the
load. The default weight is 1, and the maximal value is 256. A value of 0
means the server will not participate in load-balancing but will still accept
persistent connections. If this parameter is used to distribute the load
according to server's capacity, it is recommended to start with values which
can both grow and shrink, for instance between 10 and 100 to leave enough
room above and below for later adjustments.