Issue in setting partition size in Dask dataframe while reading from DB

I am trying to read from table and set the size of each chunk to 256MB without specifying divisions and partition. But when I checking the size of each partition it is less than 60 MB

Minimal complete code for stimulating issue:

Generate Data:

import string
import sqlite3

from itertools import permutations
def create_table(cur):    
    query = '''create table sample(id int, name text)'''
    cur.execute(query)
c = 1
def get_chunk(n, chunk_size):
    chunk = []
    global c
    for d in permutations(string.ascii_uppercase, n):
        t = c , ''.join(d)
        chunk.append(t)
        c += 1
        if len(chunk) >= chunk_size:
            yield chunk
            chunk = []
    return chunk

def insert_data(cursor, n, chunk_size): 
    for chunk in get_chunk(n, chunk_size):
        cur.executemany('insert into sample values (?, ?)', chunk)
conn = sqlite3.connect('test.db')
cur = conn.cursor()
create_table(cur)
conn.commit()
insert_data(cur, 6, 1000)
conn.commit()

For processing

import dask.dataframe as dd
df = dd.read_sql_table('sample', 'sqlite:////path/to/test.db', 'id', bytes_per_chunk=268435456)
def fun(df):
    print(df.name.count())
    print(df.info())
df.map_partitions(fun).compute()