excel size issue

i have excel book with 2 sheet inside. 1st sheet hav 100+ rows, 2nd sheet hav 12k+ rows. both sheets record are copied frm other excel book month by month. currently it is 4454KB in size, n it is having difficulty to open nor copy record nor save the file. the CPU always surge to 100% and with no responding.
i hav move both sheet to new book, and it is working fine now, any tricks or tips to avoid such scenario happen again as i hav lots of excel book tht are facing the same problem.
many thks