Meaning no offence but weighing a liner down is a poor fix and in extreme cases can tear the liner. You would be better served by filling the pond so that the water level within the liner is above the ground-water/water-table or as a not so good alternative set up permanent drainage to lower the water table.
If you liner is bulging inwards because of the water table etc (ballooning) it means that the water table is to all intents and purposes level with the pond water, as the water table rises the liner crushes to keep the pond level with the water table. If the rocks prevent sections of the liner from crushing and cause taut sections in the liner the forces and tension in the liner can become large.
For this reason all my ponds are slightly raised with the water level in each pond ABOVE the surrounding ground. There is no way, barring a surface flood, that the water table can rise to be level with the pond and my liners are always pushed out against the soil.