Use size of current block for doubling, not total

src/arena_allocator.cpp

@@ -376,10 +376,9 @@ void ArenaAllocator::add_block(size_t size) {
 }
 
 size_t ArenaAllocator::calculate_next_block_size(size_t required_size) const {
-  size_t doubled_size = total_allocated() * 2;
+  size_t doubled_size = (current_block_ ? current_block_->size : 0) * 2;
   doubled_size =
       std::min<size_t>(doubled_size, std::numeric_limits<uint32_t>::max());
-
   return std::max(required_size, doubled_size);
 }